Method of making a nanostructured electrode

ABSTRACT

A method of making a nanostructured electrode comprising depositing a self-assembled monolayer on a substrate, depositing a catalyst nanoparticle covalently bonded to a ligand, and depositing a material capable of binding to the self-assembled monolayer. The method includes depositing on a conductive electrode substrate a catalytic nanoparticle stabilized by a covalently-bound ligand bearing a peripheral functional group and depositing a material capable of binding to the peripheral functional group, wherein the conductive electrode substrate is chemically modified to create a surface functional group capable of supporting multilayer deposition. The method can include covalent grafting of a functional group to create an initial layer of positive charge on the surface, depositing a platinum nanoparticle stabilized by negatively-charged ligands onto the functional group, and providing a polymer component. The ligand can have a peripheral functional group that has a charge opposite to or chemical reactivity amenable with that of the self-assembled monolayer. The material capable of binding to the peripheral functional group is such that successive layers of the catalytic nanoparticle within a multilayered system are bridged by the material. The material can be selected from semiconductors, RuO 2 , ITO, TiO 2 , surface oxidized carbon colloid, polyoxometalate, and metal ions.

BACKGROUND

Fuel cells are simple devices capable of continuously converting storedchemical energy into electricity. In general, a fuel cell comprises apair of electrodes separated by a semi-permeable electrolyte membrane.At one of the electrodes, the anode, oxidation of input fuel occurs. Thefuels used depend on the type of fuel cell system as described below andinclude, but are not limited to, materials such as glucose, methanol,ethanol, hydrogen, formic acid, carbon monoxide, and simple hydrocarbonslike methane, propane, or butane. The electrons extracted from the fuelare transferred as electric current through an external circuit to thesecond electrode, the cathode, where an input oxidant iselectrochemically reduced. Typically, oxygen is the oxidant and it isreduced in a four-electron process to water. The semi-permeable membranefunctions to separate the fuel and oxidant. In addition, transfer ofprotons or other ions through the semi-permeable membrane ensures chargebalance and completes the circuit.

Although a variety of different systems have been developed, fuel cellscan be generally classified as one of three types based on theelectrolyte membrane type and power output operating conditions.Biological fuel cells employ electrodes modified with enzymes ormicrobes that function as the electrocatalysts. Although these cellsusually employ simple ion exchange membranes, cells that require nomembrane can also be fabricated. In these cases, cross-reactions of theanolyte fuel and catholyte oxidant with the opposite electrodes areprevented due to the high specificity of the bioelectrocatalyticreactions at the electrodes. Net chemical reactions for these cells areusually quite simple, for example, eventual oxidation of glucose tocarbon dioxide with the corresponding reduction of oxygen to water, andpower outputs are small, typically ranging from microwatts tomilliwatts.

Fuel cells at the other extremum of power output generally utilizeceramic-based or solid electrolyte membranes, operate at temperaturesgreater than ˜600° C., and can reach power output levels of kilowatts tomegawatts with fuel power conversion efficiencies exceeding 40%. Forexample, solid oxide fuel cells operating at temperatures exceeding˜800° C. utilize a solid oxide electrolyte material that transportsoxide anions through the membrane to react directly with fuel atefficiencies approaching 60%. Likewise, molten carbonate fuel cellsutilize a porous ceramic membrane containing a mixture of moltencarbonates as an electrolyte and can reach similar operatingefficiencies. Both types of cells can utilize a variety of fuels and,with proper operating configurations, achieve power outputs approaching˜100 megawatts.

Polymer electrolyte membrane (PEM) fuel cells, also known as protonexchange membrane fuel cells, represent the third general class of fuelcell systems. PEM fuel cells use a solid polymer as the electrolytemembrane in combination with porous carbon electrodes containing Ptcatalyst. Nafion® and related ionomers having good proton conductivitiesare usually used as the polymer electrolyte membranes. These systemstypically generate power at levels ranging from watts to kilowatts andoperate at temperatures ranging from ˜70° C. to ˜200° C. Their lightweight, durability, and respectable power densities compared to othertypes of fuel cells make them attractive candidates for both portable(e.g., automotive) and stationery (e.g., home power) applications. Amajority of the research in this area involves fuel cells using eithermethanol or hydrogen as fuels. PEM fuel cells using methanol as fuel andgenerating power via a methanol oxidation reaction (MOR) will hereafterbe referred to as direct methanol fuel cells (DMFCs), while those fueledby hydrogen and generating power via the oxidation of hydrogen will belabeled as hydrogen fuel cells (HFCs).

As a liquid, methanol is a more attractive fuel than hydrogen, at leastfor automotive applications, since it is more readily handled andtransported using the existing petroleum hydrocarbon infrastructure.Unfortunately, oxidation of methanol in a PEM fuel cell invariablyproduces carbon monoxide as an intermediate oxidation product, which canpoison the Pt catalyst and significantly reduce the power output of thecell. In contrast, hydrogen presents obvious dangers regarding storageand handling but is oxidized cleanly in a fuel cell to protons, whichare ultimately captured as water.

PEM fuel cell electrodes are heterogeneous supported catalyst structureswhose electrocatalytic activities are greatly affected by themicroenvironment surrounding the catalyst particles. Electrodes areusually fabricated by intimately mixing a colloidal Pt electrocatalyst,together with a small amount of Nafion® or other ionomer, in anelectrically conductive porous Vulcan carbon matrix. The resultingmixture is usually applied as thin layers to both sides of a solidNafion® film to prepare the separate anode and cathode electrodes, whichare fixed by hot-pressing or related techniques to complete the membraneelectrode assembly (MEA).

The power output available from a particular PEM fuel cell type and itspower conversion efficiency are functions of the structure andcomposition of the MEA. Consider, for example, a HFC system. Duringoperation of the cell, electrocatalysis is thought to occur mostefficiently at a triple phase boundary (R. O'Hayre, D. M. Barnett, F. B.Prinz Electrochem. Soc., 152, A439 (2005)), where H₂ fuel contacts thejunction formed by a colloidal metal catalyst particle with the ionomerand carbon support. Specifically, the hydrogen oxidation reaction (HOR)occurring at the anode (eq. (1)) is thought to be facilitated at thetriple phase boundary by efficient removal of the electron and protonproducts from the catalytic particle sites by the carbon support andionomer, respectively, minimizing the possibility of a reverse reaction.Likewise, enhanced transport of electrons and protons via the conductivecarbon and ionomer species, respectively, to catalytic particle sites attriple phase boundaries has been proposed to facilitate the oxygenreduction reaction (ORR) occurring at the cathode (eq. (2)). Similarmodels have been proposed for operation of DMFCs. Consequently,optimization of the MEA structure via changes in fabrication materialsand techniques to maximize the occurrence of such triple phaseboundaries represents a continuing focus for research to improve fuelcell performance.

Anode: H₂→2H⁺+2e ⁻  (1)

Cathode: O₂+4H⁺+4e ⁻→2H₂O  (2)

Remediation of inefficiencies specifically associated with theproperties and performances of the polymer electrolyte membrane and thePt nanoparticle catalysts to improve fuel cell performance comprise twoadditional important research areas. For example, while Nafion® polymerelectrolyte membranes efficiently transport protons required forsuccessful cell operation, the internal electrical resistance of thesesemi-permeable membranes and fuel crossover through them can reduce cellperformance. The electrochemical performance of Pt catalysts is alsolimited, especially at the cathode where the ORR suffers from slowkinetics requiring high overpotentials and Pt loadings too high forviable commercial use. Improvements in polymer electrolyte membranes todate have focused primarily on the use of new materials and modificationof membrane structures to address PEM resistance and fuel crossoverissues. Current strategies for improving the electrocatalytic activityof Pt-based catalysts mainly consist of combining Pt with othertransition metals, replacing Pt altogether with other less expensivemetals, or tailoring the Pt particle size to control the relativefraction of Pt surface atoms.

While these efforts have yielded considerable improvements in PEM fuelcell performance with regard to power density, efficiency, durability,and stability, further efforts and new research paradigms are stillrequired to realize commercial systems capable of competing economicallywith current power sources. In this disclosure, we present a newparadigm for the development of PEM fuel cells having superiorperformance characteristics based on the fabrication of nanostructuredelectrode architectures using well-defined Pt nanoparticle (NP)catalysts whose electrocatalytic activities are determined via strictcontrol of particle morphology and surface functionalization.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: LBL Multilayer Electrode Fabrication Scheme. The example showsmultilayer fabrication using Pt NPs stabilized by negatively-chargedligands (Y-shape structures) and PAH polycation. Substrate is a glassycarbon electrode (GCE) functionalized by a cationic aminophenyl (APh)monolayer (goalpost structures). The X⁻=Cl⁻, ClO₄ ⁻, and related anions.The rinse steps are not shown.

FIG. 2: Representative Structures of Some Polyelectrolytes Useful forthe Fabrication of Multilayered Electrode Architectures.

FIG. 3: Structures of Nitrogen Ligands Showing Numbered Positions forSubstituents.

FIG. 4: Triarylphosphine Ligand Structures. The core triphenylphosphineligand with substituent positions numbered is shown in the leftmostposition in the upper row. Structures and abbreviations for somespecific ligands bearing water soluble, charged substituents in themeta- (i.e., 3- or 5-) or para- (i.e., 4-) positions on each aromaticring are also shown. All structures shown are the protonated forms ofeach ligand.

FIG. 5: Structures of Other Phosphine Ligands Tested as Stabilizers forPt NPs.

FIG. 6: ³¹P NMR Spectra of TPPTP-Pt NP Dispersions in D₂O. a) Fresh; b)3 weeks old.

FIG. 7: XPS spectra for TPPTP-Pt NPs on grafted carbon paper showing the(a) Pt 4f region; (b) P 2p core levels. Part (c) shows the P 2p corelevel of the free TPPTP ligand on grafted carbon paper.

FIG. 8: a) HRTEM of TPPTP-Pt NPs; b) TPPTP-Pt NP size distributionhistogram. The average NP size is 1.7 nm±0.5 nm.

FIG. 9: HRTEM electron diffraction pattern for the TPPTP-Pt NPs.

FIG. 10: EXAFS results for TPPTP-Pt NPs. a) R-space fit. Path Pt—Pt:N=4.0, R=2.64 Å, E_(o)=−2.3 eV, σ²=0.014 Å². Path Pt—O: N=1.1, R=1.95 Å,E_(o)=−3.7 eV, σ²=0.003 Å²; b) Comparison of two-shell fit andexperimental data showing the magnitude and imaginary part of the FT inR space (FT of k²χ over the range 2<k<11).

FIG. 11: Room temperature ¹⁹⁵Pt solid state NMR of TPPTP-Pt NPs. Datawas acquired on a point by point basis with the two solid linesrepresenting deconvoluted peak fitting. The relative ratio of the peaksdue to the surface Pt atoms, S, to those of the underlying, or bulk,atoms, B, suggest that these particles are approximately 1.0 nm indiameter.

FIG. 12: Some UV-visible spectra of EDA/TPPTP-Pt NP/(PAH/TPPTP-PtNP)_(n-1) multilayer films deposited at pH 2 on fused silica. In orderof increasing absorption intensity, spectra are shown for n=6, 10, 16,and 20. Inset: Linear plots of absorbance at 223 nm (circles) and 300 nm(squares) vs. number of TPPTP-Pt NP layers. Slope=0.0361;Intercept=0.105; R²=0.9992 at 223 nm. Slope=0.0244; Intercept=0.098;R²=0.9969 at 300 nm.

FIG. 13: Pt loading (μg_(Pt)/cm²) as measured by RutherfordBackscattering Spectroscopy versus number of (TPPTP-Pt NPs)_(n) (n=1-6)layers for separate APh-modified GCEs coated with TPPTP-Pt NP/PAHmultilayers. Slope=1.217 μg·cm⁻²; Intercept=0.6733 μg·cm⁻²; R²=0.9974.

FIG. 14: RDE voltammetry for the HOR for electrodes bearing 1, 2, and 3TPPTP-Pt NP layers in H₂-saturated 0.1 M HClO₄ at 60° C., 1600 rpm, and20 mV·s⁻¹ sweep rate. Inset: Plot of i_(d) versus ω^(1/2) for the HOR at0.4 V.

FIG. 15: Tafel plots for the HOR for electrodes bearing 2 and 3 layersof TPPTP-Pt NPs in H₂-saturated 0.1 M HClO₄ at 60° C.

FIG. 16: a) RDE voltammetry for the ORR at multilayer electrodes having2, 3, 4, 5, and 6 TPPTP-Pt NP layers in O₂-saturated 0.1 M HClO₄ at 60°C., 1600 rpm, and a 20 mV·s⁻¹ sweep rate; b) Mass-specific activities at0.9 V versus layers of TPPTP-Pt NPs (2-6 layers).

FIG. 17: RDE voltammetry at multilayer electrodes having 2, 4, and 6TPPTP-Pt NP layers under Ar in 0.1 M HClO₄ at 60° C., 1600 rpm, and a 20mV·s⁻¹ sweep rate.

FIG. 18: Levich plot of the ORR at electrodes having 2, 3, 4, 5, and6-layer films of TPPTP-Pt NPs in O₂-saturated 0.1 M HClO₄ at 60° C. at0.35 V.

FIG. 19: Tafel plots for data in FIG. 16 (results are shown only for 4,5 and 6 TPPTP-Pt NP layer electrodes).

FIG. 20: Plot of 2-PVP absorbance at 260 nm for theFS-EDA/(Nafion®/2-PVP)_(x) (x=0-3) multilayers vs. number of 2-PVPlayers.

FIG. 21: Structure of theFe(II)[4,7-(m,p-sulfonato-phenyl)₂-1,10-phenanthroline]₃ ⁴⁻ Anion.

FIG. 22: UV-visible Absorption Spectra. EDA-coated FS slides treated forvarious times with Nafion® solution, followed byNa₄Fe(II)[4,7-(m,p-sulfonatophenyl)₂-1,10-phenanthroline]₃ solution, asdescribed in Example 8. Spectra in order of decreasing intensity at ˜290nm (and 540 nm) correspond to Nafion® solution treatment times of 0 sec,15 sec, 30 sec, 2 min, and 15 min.

FIG. 23: XPS Spectra of Pt 4f (top) and F 1s (bottom) for AdjacentTPPTP-Pt NP and Nafion® Layers on APhS-coated Si wafers. Conditions:TPPTP-Pt NP (0.3 mg/mL in 0.01 M HCl/0.01 M NaCl (aq) solution, 24hours); Nafion®-OH Solution (0.01 M HCl (aq) solution, 30 minutes).

FIG. 24: Comparison of ORR Voltammograms for a GCE-APh/TPPTP-PtNP/(Nafion®/PAH/TPPTP-Pt NP)₃ electrode (Curve “a”) and aGCE-APh/TPPTP-Pt NP/(PAH/TPPTP-Pt NP)₃ electrode (Curve “b”). Parametersare identical to those described in FIG. 16 and Example 23.

DESCRIPTION

As mentioned in the preceding section, current research aimed atimproving fuel cell efficiencies focuses primarily on one of threeareas: (1) formulation and processing changes associated with MEAfabrication in order to increase the fraction of triple phase boundarysites attributed to enhanced electrocatalytic performance; (2)modification of Pt NP electrocatalysts via control of the nanoparticlesize, alloying with other metals, or use of alternative metals to lowercosts and increase catalytic efficiencies; and (3) modification of thestructure and composition of the polymer electrolyte membrane todecrease internal cell resistance, promote ion (usually proton)conductivity, and minimize fuel crossover. Described here is a generalmethod for the fabrication of nanostructured three-dimensional electrodearchitectures, using ligand-stabilized Pt NPs as electrocatalysts, thatexhibit state-of-the-art efficiencies as measured by Pt mass-specificactivity, i_(m) (vide infra), at 0.9 V for the ORR in PEM fuel cells (H.A. Gasteiger, S. S. Kocha, B. Sompalli, F. T. Wagner Appl. Catal.B-Environ., 56, 9 (2005)). Consequently, the approach embodies andcombines both control of MEA electrode fabrication and Pt NP activity,rather than modification of the ionomer membrane. However, as discussedbelow, proper implementation of the methods can also indirectly yieldfurther fuel cell efficiency improvements due to decreased fuelcrossover through the ionomer membrane.

The approach involves fabricating structured electrode architecturesexhibiting superior electrocatalytic activities via layer-by-layer (LBL)deposition techniques (G. Decher Science, 277, 1232 (1997)), using PtNPs whose physicochemical properties, including electrocatalyticactivity, water solubility, net charge, and nanoparticle dispersionstability, among others, can be tailored via the judicious choice of theligand(s) covalently coordinated to the NP surface (vide infra). Ageneralized scheme for fabrication of structured electrode architecturesusing said tailored Pt NPs is shown in FIG. 1.

The fabrication of an electrode via a LBL method usually exploitsattractive electrostatic interactions between oppositely-chargedcomponents to facilitate film deposition. For example, FIG. 1illustrates an example of one common deposition protocol in which thepolymer component is a polycation such as polyallylamine hydrochloride(PAH). Likewise, the conductive glassy carbon electrode (GCE) substrateused for the deposition of the multilayer has been electrochemicallymodified to create the proper surface functional groups needed tosupport multilayer growth, for example in this case via covalentgrafting of a protonated aminophenyl (APh) functional group to create aninitial layer of positive charge on the surface (M. Delamar, R. Hitmi,J. Pinson, J. M. Savéant J. Am. Chem. Soc., 114, 5883 (1992)).Therefore, the ligand-stabilized Pt NP component must possess sufficientwater solubility and a net negative surface charge under thesedeposition conditions in order to facilitate electrostatic adsorption.This requires that the ligand coordinated to the Pt NP surface containas a portion of its structure at least one negatively-charged functionalgroup that does not bind the Pt surface. Furthermore, saidnegatively-charged functional group(s) must not interfere with thebinding of the ligand to the Pt surface by another functional group orgroups within the same ligand capable of covalently binding to the Ptsurface. For systems satisfying these conditions, immersion of themodified GCE substrate in the aqueous anionic ligand-stabilized Pt NPdispersion and the aqueous polycationic PAH solution in alternatingfashion, with aqueous rinsing after each treatment, leads to build up ofmultilayer electrode architectures comprising alternating layers of PtNPs and polyelectrolyte according to FIG. 1.

There are, of course, other possible alternatives for multilayerfabrication using the general method of FIG. 1. For example,multilayered electrode structures can clearly also be fabricated usingPt NPs stabilized using a covalently coordinated cationic ligand, incombination with a polyanion as the film components. FIG. 2 illustratesstructures of some representative polycations and polyanions. Likewise,FIGS. 3 and 4, as discussed below, show representative structures ofsome ligands useful for stabilizing Pt NPs. Note that the materialsshown in FIGS. 2-4 are meant solely as examples and do not represent anexhaustive listing of polyelectrolytes and ligands, nor are they meantto limit the scope of the invention. Multilayers stabilized by forcesother than electrostatic attraction can also be fabricated. For example,multilayers have been fabricated using separate components, such ascarboxylic acids and amides, having the abilities to formhydrogen-bonded association complexes. Similar multilayers stabilized byhydrogen bonding interactions can be fabricated using Pt NPs stabilizedby ligands bearing pendant carboxylic acid groups as one component, withpolyacrylamide as the second component of the film.

The choice of ligand can also affect the electronic properties, andtherefore the electrocatalytic ability, of the Pt NP. Another aspect ofthe approach involves tailoring the physicochemical properties,including electrocatalytic activity, water solubility, net charge, andnanoparticle dispersion stability, among others, of Pt nanoparticles viathe judicious choice of the ligand(s) covalently coordinated to the Ptparticle surface. The use of coordinated ligands as described herein toprepare well-defined Pt nanoparticle systems and control theirphysicochemical properties has not been exploited as a means to improveelectrocatalytic activities of fuel cell electrode systems.

However, ligands coordinated to single metal atoms are well known toaffect activity of the metal as a homogeneous catalyst. Limited work hasalso shown that the structure of a ligand coordinated to a metalnanoparticle can alter the selectivity of a chemical reaction, favoringone product over another (G. Schmid, V. Maihack, F. Lantermann, S.Peschel J. Chem. Soc.; Dalton Trans., 589 (1996)). Coordinated ligandsnot only can affect the selectivity of metal nanoparticles, they canalso influence the electronic properties of the metal nanoparticle core(H. Modrow, S. Bucher, J. Hormes, R. Brinkmann, H. Bönnemann J. Phys.Chem. B, 107, 3684 (2003)). Even small changes in the protecting ligandshell can lead to varying electronic properties of the metal core, ashas been observed for various ligand-stabilized Co, Pd, Fe, Ru, and Ptnanoparticles using surface-sensitive spectroscopic techniques such asEXAFS, XANES, and XPS. Such changes can have profound effects on thecatalytic activity of the metal nanoparticle. For example, chemisorptionof carbon monoxide to the surface of a Pt nanoparticle is known to blockcatalytic sites for oxidation of methanol or hydrogen in PEM fuel cells,greatly diminishing their power output (S. Yamazaki, T. Ioroi, Y.Yamada, K. Yasuda, T. Kobayashi Angew. Chem. Int. Ed., 45, 3120 (2006)).Likewise, chemisorption of alkylthiol ligands is well known to poison Ptnanoparticles as electrocatalysts for the ORR(H. Ye, R. M. Crooks J. Am.Chem. Soc., 127, 4930 (2005)).

These observations clearly demonstrate the ability to tune theelectronic, and consequently the electrocatalytic, properties of the Ptnanoparticles by changing the protecting ligand shell coordinated to thePt nanoparticle surface. They also indicate that ligands having verystrong π-acceptor or 1-donor properties that lead to strongchemisorption of said ligands at one or more atomic Pt sites on the PtNP surface can inhibit electrocatalysis and are usually not preferred asstabilizing ligands. Preferred Pt NP stabilizing ligands are thoseligands having more moderate π-acceptor and/or σ-donor properties, asare well-known to inorganic chemists and others skilled in the art ofthe design of transition metal catalysts. Among these are derivatives of1,10-phenanthroline, 2,2′:6′,2″-terpyridine, and 2,2′-bipyridine, whosegeneral structures and ring numbering conventions are shown in FIG. 3,preferably having one or more substituents at the 3, 4, 5, 6, 7, and/or8 positions, the 4, 5, 4′, 4″, and/or 5″ positions, and the 4, 5, 4′,and/or 5′ positions, respectively. Preferred substituents at these sitesinclude electron donating or accepting functional groups, not containingsulfur having a formal oxidation state <+4, with Hammett σ constants inthe range ˜0.85<σ<˜+0.85. Some specific examples of typical electrondonating and accepting substituents have been described by Della Cianaand co-workers (L. Della Ciana, W. J. Dressick, D. Sandrini, M. Maestri,M. Ciano Inorg. Chem., 29, 2792 (1990)) and Hino and co-workers (J. K.Hino, L. Della Ciana, W. J. Dressick, B. P. Sullivan Inorg. Chem., 31,1072 (1992)), both of which are incorporated herein in their entirety,and include —NO₂, —CN, —CF₃, —CO₂R (R=H or alkyl group containing 4 orfewer carbon atoms), —C₆H₅, -halogen, -alkyl (containing 4 or fewercarbons atoms), —OR (R=alkyl group containing 4 or fewer carbons atoms),and —NR₂ (R=H or an alkyl group containing 4 or fewer carbons atoms),among others. Said ligands should also contain at least one chargedsubstituent to provide water solubility for and electrostaticallystabilize the Pt NPs against agglomeration; said preferred chargedsubstituents include —PO₃ ²⁻, —SO₃ ⁻, or —COO⁻. Derivatives of1,10-phenanthroline, 2′:6′,2″-terpyridine, and 2,2′-bipyridine havingsubstituents in the 2 and/or 9 positions, 6 and or 6″ positions, and 6and/or 6′ positions, respectively, are also useful provided that thesubstituent is not sufficiently bulky to prevent bonding of the pyridylN groups to the Pt surface because of steric hindrance effects. Ingeneral, smaller substituents such as methyl and carboxyl groups, whichare known by those skilled in the art of synthesis of transition metalcomplexes to form coordinatively-saturated complexes with an individualtransition metal atom are favored, whereas bulky substituents such astertiary butyl or neopentyl groups that severely inhibit or excludecoordinatively-saturated complex formation with an individual transitionmetal atom are disfavored. Derivatives of 2′:6′,2″-terpyridine and2,2′-bipyridine having substituents in the 3, 3′, 5′, and/or 3″positions and 3 and/or 3′ positions, respectively, are less favoredbecause unfavorable steric interactions between nonhydrogen substituentsin these positions and the adjacent pyridyl ring can diminish or inhibitbinding of the ligand to the NP surface.

Derivatives of triphenylphosphine, whose general structure and ringnumbering conventions are shown in FIG. 4, also exhibit sufficientlymoderate π-acceptor and/or α-donor properties and can represent anotherpreferred class of ligands for the purpose of our invention. In thiscase, charged species such as —PO₃ ²⁻, —SO₃ ⁻, or —COO⁻ can be preferredligand substituents. At least one such substituent, covalently attachedto the meta-positions (i.e., 3, 3′, 3″, 5, 5′, and/or 5″ positions)and/or more preferably the parapositions (i.e., 4, 4′, and/or 4″positions) of the phenyl rings of triphenylphosphine core structure ofFIG. 4 may be required to provide a net negative or positive ligandcharge and water solubility for use in this invention. It is noted thatthe presence of these or any other sterically hindered substituents inthe ortho-positions (i.e., 2, 2′, 2″, 6, 6′, and/or 6″ positions) of thephenyl rings of the triphenylphosphine core structure are usuallystrongly disfavored for the purpose of this invention in that stabilizednanoparticles are usually not formed or, if formed, are often unstable.For example, attempts to form ligand-stabilized Pt NPs using stericallyhindered TXTPS (note FIG. 5) according to these protocols, as describedin the Example 7 below, led solely to formation of the TXTPS oxide insolution. Likewise, the trialkylphosphines TCEP and DCETMA shown in FIG.5, which are stronger 1-donors and more basic ligands thantriarylphosphines, were oxidized to the corresponding phosphine oxideswithout binding to the Pt NPs, as described in Example 7, withprecipitation of bulk Pt black also noted for the DCETMA ligand.

It can be noted here that the general concept of LBL fabrication ofmultilayered electrodes using Pt NPs and polyelectrolytes, such as thoseshown in FIG. 2, has previously been described in the scientificliterature. However, in each of the literature cases reported the Pt NPshave been stabilized by non-covalent adsorption of a water solublepolyelectrolyte or other protecting agent via electrostatic, hydrogenbonding, and/or van der Waals interactions, rather than covalent bindingof a ligand species as described herein. For example, several groupshave prepared Pt NPs encapsulated by adsorbed polyacrylate (PAA) andelectrostatically adsorbed them directly to a substrate surface or inalternating fashion together with poly(diallyldimethylammonium)chloride(PDDA) to fabricate single or multilayered electrodes, respectively (P.Karam, Z. G. Estephen, M. El Haraketh, M. Houry, L. I. HalaouiElectrochem. Solid-State Lett., 9, A144 (2006)). Adsorption of materialscontaining carbonyl groups, such as comprise PAA, has been ascribed toweak interactions between the Pt surface and non-bonding lone pairelectrons of carbonyl groups (L. Qui, F. Liu, L. Zhao, W. Yang, J. YaoLangmuir, 22, 4480 (2006)). Others have utilized Pt NPs coated byphysisorbed PDDA (M. Pan, H. L. Tang, S. P. Jiang, Z. Liu Electrochem.Comm., 7, 119 (2005)), which lacks any functional groups capable ofcovalently binding Pt, in combination with PAA in similar fashion.Crooks and co-workers (H. Ye, R. W. J. Scott, R. M. Crooks Langmuir, 20,2915 (2004)) have described the preparation of dendrimer-encapsulated Ptand Pd nanoparticles, in which metal nanoparticles are stabilized bynon-covalent interactions with lone pair electrons from tertiary amines,and their use to fabricate modified electrodes on GCE surfaces.Likewise, Dong and coworkers have utilized Pt NPs stabilized by aphysisorbed citrate species, together with cationic cobalt porphyrins(instead of polycations), to fabricate supramolecular electrodesarchitectures (M. Huang, Y. Shao, X. Sun, H. Chen, B. Liu, S. DongLangmuir, 21, 323 (2005)). More recently, Farhat and Hammond describedthe preparation of Pt nanoparticles stabilized by physisorbedpolyaniline (PANI) (T. R. Farhat, P. T. Hammond Chem. Mater., 18, 41(2006)), as well as Pt-loaded carbon colloids stabilized by polymerssuch as polyethyleneimine (PEI), poly(2-acrylamido-2-methyl-1-propanesulfonic acid (PAMPS), PAA, and PDDA (T. R. Farhat, P. T. Hammond Adv.Func. Mater., 16, 433 (2006)), and their use for fabrication ofmultilayered electrode structures. While the resulting electrodes inthese examples have been shown active for the HOR and/or ORR, there areno reports of electrode electrocatalytic efficiencies comparable tothose noted here (note Example 23).

Although the use of covalently-bound ligands to stabilize the Pt NPsrepresents one factor controlling the electrocatalytic activity ofcomposite Pt NP-polyelectrolyte multilayer electrode assemblies, thethree-dimensional multilayer architecture can also be important. The useof a LBL multilayered electrode structure confers the ability to tuneelectrode properties such as porosity/permeability, stability, andconductivity (both electronic and ionic) that can influence theelectrocatalytic ability of the electrode assembly. For example, thethickness and porosity of polyelectrolyte multilayer films have longbeen known to be functions of not only the chemical structures of thecomponent polyelectrolytes, but also the assembly conditions. Inparticular, the association of ionic components of salts or acids withoppositely-charged functional groups on polyelectrolytes has beenpostulated to lead to thicker multilayer films during and aftermultilayer assembly by minimizing, screening, or disruptingelectrostatic attractions between oppositely-charged polyelectrolytefunctional groups, allowing the polyelectrolytes to adopt coiled ratherthan linear conformations within the multilayer films. Deposition offilm components at elevated temperatures can produce similar effects.Corresponding changes in film permeability associated withpolyelectrolyte layer thickness have been observed and, for ourcomposite Pt NP-polyelectrolyte multilayers, provide a convenient meansto control both reactant mass transport properties and conductivity(e.g., electron transfer rates) between adjacent layers of Pt NPs withinthe electrode film.

For multilayers built using properly functionalized components, filmstability/durability/adhesion can be enhanced and porosity/permeabilitycan also be controlled via chemical crosslinking of adjacent filmlayers. For example, Bruening and coworkers have demonstrated thatsimple heating of PAH-PAA multilayers can lead to partial crosslinkingand film stabilization via amide formation during reaction of freecarboxylic acid and amine sites, as well as changes in filmporosity/permeability (J. L. Stair, J. J. Harris, M. L. Bruening Chem.Mater., 13, 2641 (2001)). Cross linking can also be accomplished byconversion of a portion of the carboxylic acid groups of the PAA towater soluble N-hydroxysuccinimide esters, as is well known to organicchemists, prior to use of the polyelectrolyte to fabricate themultilayer. During or after multilayer fabrication, reaction of theactive ester with a portion of the primary amines from the adjacent PAHlayers leads to crosslinking via covalent amide bond formation.Similarly, use of PAA solutions having ˜2.5<pH<˜4.5 near or below thepK_(a) of the PAA carboxylate groups (i.e., pK_(a) ˜4.5) yields PAH-PAAmultilayers having significant fractions of free —COOH groups within thefilm. Infusion of water-soluble carbodiimide (CDI)/water-solubleN-hydroxysuccinimide (NHS) solution into such a multilayer can activatethe free —COOH groups and promote amide crosslinking with availableamine sites. For hydrogen-bonded multilayer systems, such as thoseformed by interactions between acrylic acid and acrylamidefunctionalized species, thermal crosslinking leading to imidization tostabilize the resulting films is also possible. Photochemicalcrosslinking reactions can also be used with properly structuredmultilayer films; for example, polycationic diazo resins are well knownto covalently crosslink with polyacrylate films during UV lightexposure. For multilayer architectures comprising appropriatelyfunctionalized ligand-stabilized Pt NP and polyelectrolyte components,such as Pt NPs stabilized by the covalently-bound carboxylicacid-functionalized TPPTC ligand (note FIG. 4) and PAH or PEIpolyelectrolyte layers, similar crosslinking reactions are available asdesired to control adhesion, durability, stability, andporosity/permeability of the multilayer electrode architecture.

The conductivity of the electrode assembly can also be controlled viathe selection and nature of the material comprising the layer separatingthe ligand-stabilized Pt NPs in our films. For example, incorporation ofan electronically conducting polymer such as PANI into films can improveelectronic conductivity relative to the example using PAH shown inFIG. 1. Likewise, polyelectrolyte ionomers such as Nafion® or PAMPS canimprove ionic conductivity of multilayer films. For example, theincorporation of Nafion® in our films permits the tuning ofionic/electronic conductivity as described in Example 31 below relativeto the PAH example illustrated in FIG. 1. The incorporation ofextraneous ions from salts or acids within the polyelectrolyte, asdescribed above, can also affect conductivity and electrode performancein this regard. The presence of readily oxidized anions, such as bromideor iodide, in the anodic compartment of a PEM fuel cell must also beavoided, as these species will also curtail reactivity and power output.Preferred salts and acids useful during the fabrication of themultilayer and/or in the fluid phase during the operation of theresulting electrode are those not readily reduced or oxidized by virtueof the magnitude of their inherent electrochemical potentials and/orkinetic limitations on their electron transfer reaction rates under theconditions at which the electrode operates. For example, perchloratessatisfy these constraints and are preferred ionic species for ourelectrode systems. Likewise, other ions having no readily accessibleelectrochemical reactivity at the electrochemical potential regionsappropriate for oxidation of methanol (i.e., MOR; DMFC) or hydrogen(i.e, HOR; HFC) and the reduction of oxygen (i.e., ORR; both DMFC andHFC), such as triflate, tetrafluoroborate, and hexafluorophosphate canalso be used.

Multilayer electrode architectures useful for PEM fuel cells and relatedapplications can also be fabricated according to our methods usingligand-stabilized Pt NPs in combination with conductive materials otherthan polyelectrolytes. For example, conductive colloids or nanoparticlesstabilized by physisorbed charged materials such as citrate, ascorbate,or surfactants such as cetylammonium chloride or sodium lauryl sulfatecan be employed with oppositely-charged ligand-stabilized Pt NPs tofabricate multilayer electrode architectures. Examples of usefulelectrodes fabricated in this manner include multilayer films comprisingoxidized carbon colloids stabilized by a polycationic PDDA coating (T.R. Farhat, P. T. Hammond Adv. Func. Mater., 16, 433 (2006)) as onecomponent with any of the anionic ligand-stabilized Pt NPs shown in FIG.4. A more preferred method, providing good electrode structuralstability and conductivity, utilizes covalent interactions between theperipheral functional groups of the ligands coordinated to the Pt NPsand a second colloid or nanoparticle component during film fabrication.For example, phosphonate and carboxylate functional groups arewell-known to chemisorb to oxides of aluminum, titanium, tin, andvarious other metals. Consequently, dispersions of conductive oxidenanoparticles, such as those of indium tin oxide, tin oxide, rutheniumoxides, or polyoxometalates stabilized by physisorbed water solublesurfactants or polymers like polyvinylpyrrolidone and related stabilizermaterials, can be used as replacements for PAH or other polyelectrolytesin the fabrication of multilayer films according to the scheme inFIG. 1. Successful film fabrication in such cases can require that theperipheral functional group of the ligand-stabilized Pt NP componentcomprises a group, such as phosphonate or carboxylate, which canchemisorb to the oxide nanoparticle surface at the water-film interfaceduring deposition and partially or completely displace the surfactant orpolymer protecting said oxide nanoparticle surface. For example, Pt NPsstabilized by coordinated TPPTP and TPPTC ligands (note FIG. 4) arepreferred for multilayer film fabrication in this manner.

Simple metal ions and complexes having multiple sites capable ofinteracting via covalent bond formation, hydrogen bond formation, and/orattractive electrostatic interaction with the peripheral functionalgroups present on the ligands coordinated to the Pt NPs can also be usedfor multilayer film fabrication. For example, Zr(IV) ions are capable ofstrongly binding multiple phosphonate groups and multilayered filmscomprising Zr(IV) and organic residues containing phosphonate groupshave been previously reported. Other high-valent ions, such as Al(III)and Fe(III), also strongly bind phosphonate residues. Other metal ionsthat bind strongly to materials having multiple available phosphonate,as well as carboxylate and sulfonate, groups useful for our inventionhave been identified by Rivas and coworkers in literature publications,the contents of which are incorporated in their entirety herein (B. L.Rivas, E. Pereira, P. Gallegos, D. Homper, K. E. Geckeler J. Appl.Polym. Sci., 92, 2917 (2004)). Consequently, solutions containing simplemetal ions such as Zr(IV) or Al(III) can be used as replacements for PAHor other polyelectrolytes in the fabrication of multilayer filmsaccording to the scheme in FIG. 1. Successful film fabrication in suchcases can require that the peripheral functional group of theligand-stabilized Pt NP component comprises a group, such as preferablyphosphonate or carboxylate, which can strongly bind via covalent,electrostatic, and/or hydrogen bonding modes to available sites on themetal ion at the water-film interface during deposition of each filmlayer. Metal ions and complexes useful during the fabrication of theelectrode multilayer architecture in this manner are those that stronglyinteract with the peripheral functional groups on the ligandsstabilizing the Pt NPs and comprise two general classes. The first classincludes bridging ions such as Al(III) and Zr(IV), which are not readilyreduced or oxidized by virtue of the magnitude of their inherentelectrochemical potentials and/or kinetic limitations on their electrontransfer reaction rates. These types of ions function primarily asadhesive layer materials binding adjacent layers of ligand-stabilized PtNPs together within the electrode assembly. The second class of bridgingions are those such as Fe(III) and Cu(II), which exhibit readilyaccessible reduction and/or oxidation potentials in solution, and cantherefore affect film conductivity by functioning as electron-holeconduction paths between adjacent Pt NPs layers whenever the operatingpotential of the electrode reaches or exceeds the redox potential ofsaid bridging metal ion. In these cases, the presence of theredox-active metal ion in the electrode multilayer structure provides anadditional factor for tuning the electrocatalytic and otherphysicochemical properties of the resultant multilayer electrodeassembly.

Multilayer electrode architectures fabricated as described hereinfunction as efficient electrocatalysts, particularly for the HOR and ORRimportant for HFCs. For example, for multilayer assemblies fabricatedusing ˜1.7 nm diameter TPPTP-stabilized Pt NPs and PAH component layerson glassy carbon rotating disk electrode (RDE) surfaces modified withcovalently-grafted protonated aminophenyl groups, fully mass-transportlimited kinetics for hydrogen oxidation are obtained in a filmcontaining just three layers of TPPTP-Pt NPs at a total Pt loading of4.2 μg/cm². Complete reduction of oxygen by four electrons is achievedwith four layers of TPPTP-Pt NPs and a total Pt loading of 5.6 μg/cm². Amaximum current density for oxygen reduction is reached with filmscontaining five Pt NP layers, corresponding to a loading of 6.6 μgPt/cm² and resulting in a mass-specific activity, i_(m), of 0.11A/mg_(Pt) at 0.9 V vs. a reference hydrogen electrode (R.H.E.). Thisactivity is comparable to the state-of-the-art i_(m) of 0.19 A/mg_(Pt)at 0.9 V reported for conventional Vulcan carbon-supported Pt (46% Pt—C)HFC electrodes (H. A. Gasteiger, S. S. Kocha, B. Sompalli, F. T. WagnerAppl. Catal. B-Environ., 56, 9 (2005)). Although the multilayerelectrodes are fabricated on low geometric surface area planar carbonsubstrates, absolute current densities approaching those observed forthicker, more porous Pt-Vulcan carbon fuel cell electrodes can beachieved by judicious choice of the multilayer components and depositionconditions. Current densities can be further increased through the useof conductive, mesoporous, high surface area carbon, semiconductor, ormetal foams as electrode supports.

Fabrication of MEAs using the multilayer electrode architecturesdescribed herein can be accomplished through either of two primaryroutes. In the first, electrodes functioning as anode and cathode areseparately fabricated on porous, conductive substrates such as carbonvia the LBL deposition method according to the general scheme shown inFIG. 1. The resulting electrodes are then pressed to contact each sideof a Nafion® ionomer membrane, such that the multilayer film liesbetween the conductive substrate and ionomer membrane in each case, tocomplete the MEA required for the PEM fuel cell. For a DMFC, use of aNafion® ionomer membrane onto which a multilayer comprising alternatinglayers of PDDA and PSS or PDDA andpoly(1-(4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido)-1,2-ethanediyl, sodium salt (PAZO) has been depositedimproves power output ˜42% by reducing methanol fuel crossover and istherefore preferred over an unmodified Nafion® ionomer membrane. MEAscan also be fabricated by multilayer deposition according to the generalscheme shown in FIG. 1 using a modified or unmodified Nafion® ionomermembrane, as appropriate, as the substrate. Deposition of Pt NPcontaining multilayers on both sides of the ionomer membranesimultaneously fabricates both electrodes. The MEA is completed bycontacting the multilayers present on both sides of the ionomer membraneby a conductive, porous, support comprising carbon or a relatedmaterial. Because a multilayer electrode architecture containing 6layers of 1.7 nm diameter TPPTP-stabilized Pt NPs and PAH is only ˜20 nmthick, the resulting MEAs benefit from generally lighter weight, smallersize, and improved flexibility, which are especially desirable forportable power applications, compared to conventional Pt-Vulcan carbonanalogs.

Having described the invention, the following examples are given toillustrate specific applications of and provide a better understandingof the invention. These specific examples are not intended to limit thescope of the invention described in this application.

EXAMPLES

Materials: Hydrogen hexachloroplatinate(IV) hexahydrate (H₂PtCl₆.6H₂O,Strem), perchloric acid (GF Smith), p-aminophenyltrimethoxysilane (APHS,Gelest, Inc.), tris(3-sulfonatophenyl)phosphine (TPPTS, Strem),poly-2-vinyl-pyridine (Linear Polymer Inc.; average molecular weight˜40,000 g/mole; Lot #09), and Sephadex LH-20 (Amersham Bioscience) wereused as received. Tetrabutylammonium tetrafluoroborate (TBA⁺BF₄ ⁻),tetrabutylammonium bromide (TBA⁺Br⁻), acetonitrile (ACN, Sure/Seal™),2-(N-morpholinoethane)sulfonic acid (MES), hydrochloric acid (37%weight), methanol, ethanol, isopropanol, poly(allylamine hydrochloride)(PAH, average molecular weight 15,000 g/mol, lot #01916BC),poly(allylamine hydrochloride) (PAH, average molecular weight range8,000 g/mole, lot #TG123713MG), potassium chloride, sodium chloride,sodium perchlorate, FeSO₄.7H₂O, Fe(NH₄)₂SO₄, CaCl₂, Eu(NO₃)₃, NiSO₄,CuCl₂, and CoSO₄, polyaniline (PANI, emeraldine base, molecular weight10,000 g/mole; lot number 15214 MB), and Nafion® (10% weight dispersionin water; ρ=1.05 g/mL; lot number 02721TC) were A.C.S. Reagent Grade orbetter and were used as received from Aldrich Chemical Company.Tris(4-phosphonatophenyl)phosphine (TPPTP),tris(4-carboxyphenyl)phosphine (TPPTC) (T. L. Schull, S. L. Brandow, W.J. Dressick Tetrahedron Lett., 42, 5373 (2001)), and(4-nitrophenyl)diazonium tetrafluoroborate were synthesized according topublished procedures. N-(2-aminoethyl)-3-aminopropyltrimethoxysilane(EDA, Gelest, Inc.) was purified by vacuum distillation (140° C., 14 mmHg) and stored under a dry nitrogen atmosphere until needed for use.Optical grade polished fused silica (FS) slides (25 mm×25 mm×1 mm) werepurchased from Dell Optics Inc. All aqueous solutions were prepared withwater purified by a Millipore Milli-Q Plus system to 18.2 MΩ-cm.Spectra/Pore® Biotech Cellulose Ester (CE) Dialysis Membrane (molecularweight cut-off ˜500 g/mole; flat width ˜16 mm; diameter ˜10 mm;volume/length ˜0.81 mL/cm) was used as received from Spectrum Labs Inc.Nuclear Magnetic Resonance Spectroscopy (NMR): Solution ¹H and ³¹P NMRspectra were recorded on a Bruker DRX 400 spectrometer and referenced toresidual ¹H signals of the deuterated water (¹H) or external H₃PO₄(³¹P). All ¹⁹⁵Pt solid-state Nuclear Magnetic Resonance (ssNMR) studieswere performed at room temperature (298K) at 11.7 T on a Bruker DigitalDMX500 spectrometer interfaced with a Silicon Graphics console runningthe XWINNMR 2.6 software package. Approximately 100 mg of the Pt NPsolid of interest was loosely packed into a 4 mm sample vial underambient conditions. Data was acquired on a point by point basis understatic conditions using a simple Hartman-Hahn solid state echo pulsesequence with a recycle delay of 25 ms and a τ of 20 μs.X-ray Photoelectron Spectroscopy (XPS): XPS spectra were acquired usinga Thermo VG Scientific Escalab 220i-XL with a monochromatic Al Kαsource. Carbon paper modified with negatively charged ligand-stabilizedPt NPs, as described in the Example 9 below, was dried overnight beforeplacing it in the UHV chamber. Measurements were performed at roomtemperature with a base pressure of 1×10⁻⁹ Torr. Survey scans wereperformed from 0 to 1400 eV binding energies and 100 eV pass energy.High-resolution scans of Pt 4f and 4d, O 1s, C 1s, N 1s, and P 2p wereacquired with 15-20 eV windows and 20 eV pass energy. The core-levelbinding energies were calibrated to the C 1s peak at 284.4 eV. The highresolution spectra were fit using Unifit software (R. Hesse, T. Chesse,R. Szargan Fresenius' J. Anal. Chem., 365, 48 (1999)). TransmissionElectron Microscopy (TEM): High resolution TEM and electron diffractionwere performed on a JEOL JEM-2200FS microscope operating at a 200 kVaccelerating voltage. Samples were prepared by placing a drop of anaqueous TPPTP-Pt NP dispersion (0.25 mg/mL in the presence of asurfactant, ca. 10 mg tetrabutylammonium bromide) on a 300-mesh coppergrid coated with continuous carbon film (Ted Pella). The samples weredried overnight in vacuo. Images were recorded using a Gatan UltascanCCD camera, and the camera constants were calibrated using gold latticeimages. Pt core diameters were measured using the Image Processing ToolKit plug in for Photoshop.Extended X-ray Absorption Fine Structure (EXAFS): EXAFS measurements ofTPPTP-Pt NPs were taken in fluorescent mode at beamline X11B at theNational Synchrotron Light Source (NSLS) at Brookhaven NationalLaboratory. The spectra were collected on a monolayer of TPPTP-Pt NPsdeposited on grafted carbon paper as described below in theElectrochemistry Section. Data analysis was performed with the IFEFFITprogram (B. Ravel, N. Newville J. Synch. Rad., 12, 537 (2005)) using anR range of 0.5 to 2.8 Å and the Fourier transform k³ (2<k<11) for thePt—Pt and Pt—O scattering paths. The data from k>11 was not useful dueto background noise in the spectra from these low-platinum films. Theatomic EXAFS contribution below R=3 Å is reduced by adjusting theR_(bkg) parameter in the Athena code of the IFEFFIT package.UV-Visible Spectroscopy: UV-vis spectra, corrected for baselinevariations using EDA-coated fused silica reference slides were acquiredusing a Varian Cary 5000 double beam spectrophotometer. EDAself-assembled monolayers were chemisorbed onto freshly cleaned fusedsilica slides using the literature procedure (M-S. Chen, S. L. Brandow,C. S. Dulcey, W. J. Dressick, G. N. Taylor, J. F. Bohland, J. H.Georger, Jr., E. K. Pavelchek, J. M. Calvert J. Electrochem. Soc., 146,1421 (1999)).Multilayer Electrode Architecture Fabrication: Multilayers werefabricated on the EDA-coated fused silica slides and glassy carbonelectrodes (GCEs) functionalized with monolayers of aminophenyl (APh)groups as described in detail in the examples below. A programmedStrato-Sequence VI® robot dipcoater (nanoStrata Inc.) was used todeposit each layer of the film on the EDA-coated fused silica slides.The treated slides were automatically triple rinsed using DI water (1min per rinse cycle) and dried in a filtered N₂ gas stream (30 s)following deposition of each film layer unless noted otherwise.Deposition times on the EDA-coated fused silica slides were typically120 min for the ligand-stabilized Pt NPs dispersion and 30 min for theoppositely-charged polyelectrolyte solution. APh-modified GCEs weresimilarly coated by sequentially hand dipping the substrate in unstirredPt NP dispersions and oppositely-charged polyelectrolyte solutions, withtriple aqueous rinses of the GCE between immersions unless notedotherwise. Deposition times for the Pt NP dispersions on GCEs wereincreased to ˜24 h, whereas immersion in the oppositely-chargedpolyelectrolyte solution remained at 30 min. The composition of themultilayer so obtained is indicated by the shorthand notation,SUB-SAM/L-Pt NP/(PE/L-Pt NP)_(n-1), where “SUB” indicates the substratematerial (i.e., fused silica, FS, or the glassy carbon electrode, GCE),“SAM” indicates the type of monolayer grafted to the substrate (i.e.,EDA or ganosiloxane for the FS slide and APh monolayer for the GCE), “L”indicates the specific stabilizing ligand covalently coordinated to thePt NP surface (e.g., note FIGS. 3 and 4), “PE” indicates theabbreviation of the specific polyelectrolyte (e.g., note FIG. 2) usedwith the L-Pt NPs to form the multilayer, and “n” is the total number ofL-Pt NP layers present following n−1 PE/L-Pt NP treatment cycles of themonolayer-functionalized substrate coated with the initial L-Pt NPlayer, “SUB-SAM/L-Pt NP”. Specific treatment conditions and notationsfor multilayer identification are given in the relevant examples below.Electrochemistry. HOR and ORR kinetics were evaluated for the multilayerelectrode architectures prepared on GCEs using a rotating disk electrode(RDE) method. Aqueous 0.1 M perchloric acid was used as electrolyte, ahydrogen impregnated palladium bead (Pd/H₂) as the reference electrode,and Au foil as the counter electrode. The RDE cell was a 250-mL volumeglass cell with a jacket for temperature control. The temperature wasmaintained at 60° C. with a circulating bath of 1:1 (v:v) mixture ofethylene glycol and water. Electrolyte solutions were saturated with H₂or O₂ for the HOR and ORR evaluation, respectively, by bubbling gas intothe solution through medium porosity glass frits. Electrode rotationrates were controlled using a Pine Instruments AFMSRX rotator. Electrodepotentials were applied using an AUTOLAB™ potentiostat. Computer controlof the potentiostat and data acquisition was performed with GPES™electrochemical software. The current density was calculated using thegeometric surface area of the glassy carbon electrode disk (0.196 cm²).The Pd/H electrode was corrected to a reversible hydrogen electrode(RHE) by measuring the potential at which a Pt∥Pt cell exhibited zerocurrent under hydrogen in the electrolyte. Platinum loadings weredetermined for each multilayer-coated GCE after electrochemicalmeasurements using Rutherford backscattering spectroscopy (EvansAnalytical Group, Sunnyvale, Calif.) on the glassy carbon electrodedirectly.

Example 1 Preparation of Preferred Pt NP Starting Material

This example describes the synthesis of the glycol-coated Pt NPs mostuseful for preparation of ligand-stabilized Pt NPs useful for ourinvention.

Pt nanoparticles stabilized by glycol and OH⁻ were prepared according toa previously reported method (Y. Wang, J. Ren, K. Deng, L. Gui, Y. TangChem. Mater., 12, 1622 (2000)). Briefly, under an inert atmosphere, anethylene glycol solution of H₂PtCl₆.6H₂O (50 mL, 1.93 mmol) was added toan ethylene glycol solution of NaOH (50 mL, 0.5 M). The resultingorange-yellow solution was heated to 160° C. for 3 h under reflux. Atransparent brown Pt colloidal solution was obtained and stored underN₂.

Example 2 Preparation of TPPTP-Stabilized Pt NPs

This example describes the preparation of ˜1.7 nm diameter Pt NPsstabilized by covalently coordinated tris-4-phosphonatophenyl phosphine(TPPTP, note FIG. 4) ligands on the NP surface.

The glycol-stabilized Pt NPs (30 mL, 0.579 mmol Pt) from Example 1 wereisolated as a brown solid via precipitation with 1.0 M HCl until a pHvalue of less than 4 was reached. After centrifugation (3000 rpm for 10min) to separate the precipitate from the supernatant, the isolated PtNPs were dispersed in a minimal amount of acetone and precipitated with1.0 M HCl, followed by centrifugation to isolate the brown Pt NPs. Thiswas repeated three times to remove excess ethylene glycol. The isolatedPt NPs were dispersed in degassed acetone (5 mL) and added to a solutionof TPPTP (0.153 g, 0.290 mmol) in degassed water (15 mL) resulting in ahomogenous brown dispersion. This dispersion was mechanically stirredunder N₂ for 1 h to allow for partial exchange of the TPPTP ligand atthe platinum surface. The solvent mixture was then removed in vacuo anddegassed water (˜10 mL) was added, resulting in a transparent brown Ptcolloidal dispersion which was left stirring overnight under N₂atmosphere. The next day, ˜0.50 mL of a solution of 30% weight NaOD inD₂O was added to the dispersion to precipitate a brown solid that wascollected by centrifugation, re-suspended in ˜10-15 mL H₂O, and allowedto stir under N₂ atmosphere to complete the ligand exchange. Completeexchange of the TPPTP ligand at the Pt NP surface required 2-3 days. TheTPPTP ligand is known to coordinate to low-valent transition metalsexclusively through the phosphine phosphorus, and the ligand exchangereaction can be monitored by solution ³¹P NMR spectroscopy. Coordinationof the TPPTP ligand to the platinum nanoparticle is evidenced by thedisappearance of the resonance for free phosphine of TPPTP at −6.5 ppmand a new resonance appearing downfield at 3.2 ppm. This is similar tothe behavior observed for phosphine ligands coordinated to palladiumNPs. However, no J_(Pt—P) coupling is observed, which is in contrast tothe findings of Chaudret, et al., who observed complex multiplets in the³¹P NMR spectrum and claim a ¹J_(Pt—P) value of 5130 Hz fortriphenylphosphine bound to 1.3 nm Pt clusters (C. Amiens, D. de Caro,B. Chaudret, J. S. Bradley, R. Mazel, C. Roucau J. Am. Chem. Soc., 115,11638 (1993)). Once no free TPPTP ligand was observed by ³¹P NMR, thesolvent was removed in vacuo. To remove the excess free TPPTP oxideformed during the ligand exchange process, the crude material wasre-dispersed in a minimal amount of water and purified by gel filtrationchromatography using Sephadex LH-20. The brown-yellow layer wascollected, and the water was removed in vacuo to give a black solid(0.117 g) which is completely redispersible in water. Multiple elementalanalyses by ICP (Robertson Microlit Laboratories) showed a Pt metalcontent in the range of 66.08 to 58.25% and P content of 5.91 to 4.47%corresponding to a Pt:TPPTP ratio of ˜7:1.

Example 3 Stability of TPPTP-Pt NPs

This example demonstrates the stability of the TPPTP-Pt NPs in aqueoussolutions used for the fabrication of the multilayer electrodearchitectures.

The stability of the TPPTP-Pt NPs prepared in Example 2 is demonstratedvia ³¹P NMR spectroscopy by the absence of fast TPPTP ligand exchangeand the inability to displace TPPTP coordinated to the Pt NP surface byalkylthiol ligands. For example, addition of free TPPTP (10 mg) to adispersion of TPPTP-Pt NPs (20 mg) in D₂O (1 mL) causes a shift of thebound TPPTP ³¹P resonance from 3.2 to 0.5 ppm, but no line broadeningfor either the free or bound phosphine resonance is observed. Additionof another 10 mg of TPPTP results in no change in the ³¹P NMR spectrum.Heating the solution to 80° C. resulted in no line broadening of eitherthe free TPPTP or TPPTP-Pt NPs resonance, consistent with a lack ofTPPTP ligand exchange for these TPPTP-Pt NPs. In a second ligandexchange experiment, we attempted to displace TPPTP bound to the Pt NPsurface by addition of an alkylthiol ligand, 2-mercaptoethanol (0.2 mL,ca. 2500 equivalents to TPPTP), to a dispersion of TPPTP-Pt NPs (20 mg)in 0.7 mL of D₂O. After six days, no displaced TPPTP was observed by ³¹PNMR, indicating that the TPPTP is tightly bound to the nanoparticlesurface.

Example 4 Conformational Mobility Within TPPTP Ligand Coordinated to PtNPs

This example shows that TPPTP ligand coordinated to the Pt NP surfaceretains its conformational mobility.

A dispersion of the TPPTP-Pt NPs isolated in Example 2 in D₂O (˜20mg/mL) exhibits the ³¹P NMR spectrum shown in FIG. 6 a. The sharp peakattributed to the phosphine P resonance of the coordinated TPPTP isobserved at 2.89 ppm. The corresponding resonance due to the phosphonateP atoms is located at 12.2 ppm. Peaks at 10.14 ppm and 38.22 ppm areassigned to TPPTP oxide by comparison with an authentic sample.Integration of the phosphine and phosphonate peaks for the TPPTP ligandcoordinated to the Pt NPs yields a PO₃ ²⁻:P ratio of ˜1:1, rather thanthe 3:1 ratio expected from the TPPTP ligand structure (note FIG. 4).The spectrum of an identical solution stored at room temperature for ˜3weeks is shown in FIG. 6 b. Peaks at 12.12 ppm and 36.71 ppm are againassigned to TPPTP oxide. The position of the phosphine P peak for thecoordinated TPPTP ligand has shifted to 0.60 ppm. In addition, thecorresponding phosphonate P peak has now coalesced and appears as theexpected singlet at 15.37 ppm. In addition, integration now yields a PO₃²⁻:P ratio of 2.78:1, as expected based on the TPPTP ligand structure inFIG. 4. The behavior shown in FIG. 6 is consistent with the presence ofphosphonate P atoms in multiple chemically and/or magneticallynon-equivalent environments, at least some of which can quench the ³¹Presonance signal of the phosphonate P, immediately following ligation ofTPPTP by the Pt NP, with subsequent conformational relaxation to asingle average chemically and magnetically equivalent environment withtime.

Example 5 Preparation of TPPTC-Stabilized Pt NPs

This example describes the preparation of ˜1.7 nm diameter Pt NPsstabilized by covalently coordinated tris-4-carboxyphenyl phosphine(TPPTC, note FIG. 4) ligands on the NP surface.

An aqueous solution was prepared by suspending 0.114 gtris-4-carboxyphenyl phosphine (TPPTC) ligand in 5 mL of N₂ degassedwater and adding 30% weight NaOD (aq) solution dropwise until all thesolid had dissolved (3-4 drops were required). The glycol-stabilized PtNPs (30 mL, 0.579 mmol Pt) from Example 1 were isolated and dispersed in15 mL acetone, as described in Example 2. The glycol-stabilized Pt NPdispersion in acetone was then slowly added to the aqueous TPPTCsolution and the resulting dispersion was stirred for 1 h under N₂atmosphere. The NPs were then isolated by centrifugation (3000 rpm for10 min), re-dispersed in 20 mL water, and stirred under a N₂ atmospherefor 3 days to complete the ligand exchange reaction. The TPPTC-Pt NPsobtained were purified by chromatography using Sephadex LH-20, asdescribed for the TPPTP-Pt NPs in Example 2. The purified TPPTC-Pt NPs(0.144 g) exhibited a ³¹P NMR phosphine resonance at 2.70 ppm.

Example 6 Preparation of TPPTS-Stabilized Pt NPs

This example describes the preparation of ˜1.7 nm diameter Pt NPsstabilized by covalently coordinated tris-3-sulfonatophenyl phosphine(TPPTS, note FIG. 4) ligands on the NP surface.

The preparation of TPPTS-Pt NPs was carried out as described in Example2 using tris-4-sulfonatophenyl phosphine (TPPTS) ligand in place ofTPPTP ligand with one change in procedure. The step involving treatmentof the Pt NP dispersion with 30% weight NaOD solution in D₂O was notrequired in this case to enhance the binding rate of the TPPTS ligand tothe Pt NP surface and was omitted. The TPPTS-Pt NPs obtained (0.104 g)exhibited a ³¹P NMR phosphine resonance at 0.52 ppm.

Example 7 Steric Hindrance and Basicity Considerations Regarding thePreparation of Pt NPs Stabilized by Coordinated Triaryl- andTrialkylphosphine Ligands

This example illustrates limitations regarding steric hindrance andbasicity of triarylphosphines and trialkylphosphines, respectively, ofligands for stabilization of Pt NPs.

The glycol-stabilized Pt NPs prepared in Example 1 were separatelytreated with various phosphines (FIG. 5) having increased sterichindrance (i.e., TXPTS) or basicity (i.e., DCETMA and TCEP) compared tothe triarylphosphines shown in FIG. 4. The general procedure was tocombine an acetone solution of the glycol-stabilized Pt NPs with anaqueous solution containing 0.5 equivalents (to Pt) of the appropriatewater-soluble phosphine under N₂ atmosphere. The exchange reactionassociated with binding of the phosphine ligand to the Pt NP surface wasfollowed by ³¹P NMR. Coordination of the ligand to the metalnanoparticles, if it occurred, was evidenced by the disappearance of theresonance of the free phosphine and a new resonance appearing downfield,as observed for TPPTP in Example 2. Neither the basic trialkylphosphineligand, TCEP, nor the sterically hindered triarylphosphine ligand,TXPTS, showed coordination to the NPs; only phosphine oxide formationwas observed. The basic trialkylphosphine ligand, DCETMA, appeared toinitially coordinate to the metal; the original resonance at −7 ppmmoved to 34 ppm, but this complex quickly decomposed to platinum blackand phosphine oxide after isolation of the solid and reconstitution inD₂O.

Example 8 Sensitivity of Electronic States of Pt NPs to LigandEnvironment

This example demonstrates the ability to alter the electronic states ofPt NPs using ligands.

TPPTS-stabilized Pt NPs were prepared by addition of solid K₂PtCl₆ (1equivalent) to ˜10 mL of a stirred aqueous solution containing NaBH₄(˜10 equivalents) and TPPTS (0.2 equivalents) under N₂ atmosphere. After˜12 h, the mixture was centrifuged and then filtered (0.4 μm porosityfilter) to remove the Pt black that had formed. Addition of ˜50 mLethanol to the filtrate precipitated a brown-black solid, which wasisolated by centrifugation. The solid was re-dispersed in ˜5 mL waterand the precipitation with ethanol was repeated twice. The solid wasthen re-dispersed in ˜3 mL water, purified via Sephadex LH-20chromatography, and isolated as described in Example 2 for the TPPTP-PtNPs. The TPPTS-Pt NPs so obtained were dispersed in D₂O (˜20 mg/mL) anda ³¹P NMR spectrum was obtained. A single phosphine P resonance wasobserved at 0.8 ppm. A second NMR spectrum was obtained followingaddition and dissolution of 40 mg solid KCl to the sample. The phosphineP peak for this sample was shifted to 3.3 ppm. No other ³¹P peaks, suchas those due to free TPPTS ligand (−5.78 ppm) or TPPTS phosphine oxide(34.7 ppm) indicative of decomposition of the TPPTS-Pt NPs, wereobserved. The TPPTS-Pt NP dispersion was then subjected to a secondSephadex LH-20 chromatography to separate the NPs from the excess KCland re-dispersed in D₂O before another ³¹P NMR spectrum was recorded.The sample again showed the original resonance at 0.8 ppm, indicative ofa reversible effect of the KCl ligand. This behavior is consistent witha reversible interaction of the KCl with available sites on the Pt NPsurface, leading to a perturbation of the energy levels of the Pt NP asmeasured by the shift in the ³¹P NMR signal of the bound TPPTS ligand.

Example 9 Method for Chemically Grafting Monolayer Films ContainingAminophenyl Functional Groups to a Carbon Surface

This example describes the procedure for electrochemically grafting afilm containing aminophenyl functional groups to the surface of aconductive carbon substrate, such as a GCE or carbon paper.

The surface of the glassy carbon electrode (GCE) or conductive carbonpaper was grafted with a monolayer of 4-aminophenyl (APh) functionalgroups according to published procedures (M. Delamar, R. Hitmi, J.Pinson, J. M. Savéant J. Am. Chem. Soc., 114, 5883 (1992)). Briefly,glassy carbon disk electrodes (5.0 mm diameter, 0.196 cm², PineInstruments), polished to a mirror finish with 0.1 μm alumina powder ona polishing cloth (Buehler), or pieces of conductive carbon paper weresuspended under Ar in a solution of freshly prepared 5 mM(4-nitrophenyl)diazonium tetrafluoroborate and 0.1 M TBA⁺BF₄ ⁻ inacetonitrile. Electrolysis at 1.1 V vs. Ag/Ag⁺ reference electrode for10 minutes resulted in the covalent attachment of 4-nitrophenylfunctional groups to the electrode surface, as confirmed by thereversible wave observed in the cyclic voltammetry in pure electrolyte.Reduction of the nitro group to an amine was achieved by applying apotential of −1.2 V for 10 minutes in a protic solution (0.1 M KCl in90:10 water/ethanol), resulting in the formation of APh functionalgroups on the GCE surface. The APh-modified GCE (GCE-APh) was immersedinto a 0.1 M HClO₄ solution for 10 minutes to protonate the amine group,resulting in a uniform positively charged surface to which thenegatively charged polyelectrolytes or ligand-stabilized Pt NPs canelectrostatically bind for multilayer film fabrication.

Example 10 Mode of Binding of TPPTP to Pt NPs

This example demonstrates that the TPPTP ligand binds to the surface ofthe Pt NPs via coordination of its phosphine P site and confirms thepresence of platinum oxide on the Pt NP surface.

To confirm the platinum-phosphorus interaction of TPPTP-stabilized PtNPs and the oxidation state of Pt, the TPPTP-Pt NPs were analyzed byX-ray photoelectron spectroscopy. FIG. 7 a shows the Pt 4f region of thespectrum before electrochemical measurements which can be deconvolutedinto two spin-orbit doublets. The more intense doublet of Pt 4f_(7/2) ismeasured at a binding energy (BE) of 71.6 eV, and although shifted frombulk platinum metal (Pt 4f_(7/2)=71.12 eV), is characteristic of Pt inthe zero-valent state for small platinum nanoparticles. The higher BEcomponent of Pt 4f_(7/2) at 72.7 eV is assigned to Pt^(II) consistentwith PtO or Pt(OH)₂ on the surface of the nanoparticle.

The P 2p signal of the TPPTP ligand on Pt was also measured (FIG. 7 b).Only one spin-orbit doublet was observed at 132.4 eV for both thesurface bound phosphine and phosphonate groups of the Pt-bound ligand.This is consistent with a shift of the phosphine P 2p peak to a higherbinding energy due to coordination to the platinum metal, resulting fromelectron donation from the phosphine to the metal. A similar shift hasbeen observed before for 1.3 nm PPh₃-Pt NPs, where the binding energy offree PPh₃ is observed at 130.9 eV, but shifts to 131.8 eV uponcoordination to platinum. To determine whether a similar binding energyshift occurs for the Pt-coordinated TPPTP, a monolayer of free TPPTPligand electrostatically bound to APh-grafted carbon paper was alsoanalyzed by XPS. FIG. 7 c shows the P 2p region of the spectrum for freeTPPTP, which now shows the expected two spin-orbit doublets at 130.6 eVfor the phosphine and 133.0 eV for the three phosphonate groups in aratio of roughly 1:3, respectively. These results are consistent withthe phosphine of the TPPTP ligand coordinated to the surface Pt atoms ofthe nanoparticle.

Example 11 Size Determination for the TPPTP-Pt NPs

This example shows that the Pt NPs stabilized by coordinated TPPTPligand have an average diameter of ˜1.7 nm.

FIG. 8 a shows a high-resolution TEM image of the isolated TPPTP-Pt NPsprepared in Example 2. The histogram resulting from measuring 214well-separated particles in FIG. 8 b displays a mean particle diameterof 1.7±0.5 nm.

Example 12 Structure Determination for the TPPTP-Pt NPs

This example illustrates the internal crystalline structure of theTPPTP-Pt NPs.

An electron diffraction pattern of the TPPTP-Pt NPs, determined inconjunction with the TEM particle size measurements from Example 11, isshown in FIG. 9. The electron diffraction pattern exhibits fourdiffraction rings which are indexed as the d spacings for the (111),(200), (220), and (311) planes of a face centered-cubic platinumstructure. This confirms the presence of a crystalline structure for theTPPTP-Pt NPs.

Example 13 Shape Determination for the TPPTP-Pt NPs by EXAFS

This example illustrates the distortion of the Pt NP structure due tochanges in the particle energy levels resulting from NP interactionswith the coordinated TPPTP ligand.

EXAFS analysis gives information concerning atomic interactions with theTPPTP-Pt NPs, as well as additional information about particle size and,together with the TEM results (Example 11) and solid-state NMR resultsdescribed in Example 14, particle shape. The EXAFS data for the TPPTP-PtNPs is shown in FIG. 10 and is best fit by the 2-shell interactions ofPt—Pt and Pt—O. FIG. 10 a shows the quality of the fit in R-space usingthe region 1.4<R<3.3 derived from χ(k) data in k-space from 2 to 11 Å⁻¹shown in FIG. 10 b. Higher multiple scattering contributions enter aboveR=3 Å and an atomic EXAFS contribution appears below R=1.4 Å. Table 1summarizes the parameters corresponding to the best fit of the data inFIG. 10, which corresponds to particle having a 2.65 Å Pt—Pt interactionwith a coordination number of N=4.0 and a Pt—O interaction with a bonddistance of 1.95 Å and a coordination number of N=1.1.

The small Pt—Pt coordination number gives indirect evidence for thepresence of TPPTP ligands bound to Pt metal on the surface of thenanoparticles. The Pt—P contribution could not be modeled directly inthe fit with the Pt—Pt and Pt—O because each interaction requires 4parameters, but only 10 parameters are allowed with the Nyquist theorem.In addition, the 2.5 Å Pt—P contribution is also difficult to separatefrom the 2.65 Å Pt—Pt contribution because of the weaker scatteringsignal and the lower number of Pt—P interactions evidenced from the 7:1Pt:P ratio obtained from the elemental analyses (note Example 2) and the1:3 P:PO₃ ²⁻ ratio for P available to coordinate to the NP surface basedon the TPPTP ligand structure (note FIG. 4). Evidence for a Pt—Pinteraction comes from the somewhat large Debye-Waller factor σ², ameasure of structural disorder, which is consistent with stronginteraction of the surface atoms with adsorbed molecules, as is theshort Pt—Pt bond length (2.65 Å vs. 2.78 Å in bulk Pt metal), whichtends to decrease with decreasing particle size and interaction withsurface-bound molecules. Others have shown that for Ru nanoparticles,4-nm particle capped with thiols had a lower coordination number than 2nm particles coated with polyol, due to the disorder introduced by thestrong thiol ligands (N. Chakroune, G. Viau, S. Ammar, L. Poul, D.Veautier, M. M. Chehimi, C. Mangeney, F. Villain, F. Fiévet Langmuir,21, 6788 (2005)). These EXAFS results suggest that the TPPTP-PT NPscomprise a crystalline Pt core of ˜0.5 nm diameter having ˜5-7 Pt atomssurrounded by a shell containing Pt atoms and PtO, which is distorteddue to strong interactions of the Pt with the coordinated TPPTP ligand.

TABLE 1 EXAFS fit parameters. Path N R (Å) E_(o) (eV) σ² (Å²) Pt—Pt 4.02.64 −2.3 .014 Pt—O 1.1 1.95 −3.7 .003

Example 14 Shape Determination for the TPPTP-Pt NPs by ¹⁹⁵Pt Solid-StateNMR

This example demonstrates that the TPPTP-Pt NPs possess flattened ortruncated cubooctahedral, rather than spherical, geometries as a resultof distortion of the surface Pt atoms by interactions with thecovalently coordinated TPPTP ligands.

The degree to which stabilizing ligands or other adsorbate moleculesinteract with the surface atoms of a platinum particle can also beobserved by solid state ¹⁹⁵Pt NMR. Unlike most non-metals, the NMR of atransition metal, such as Pt, reveals a resonance peak whose position isnot solely due to chemical shift but also has a contribution from theKnight shift. The Knight shift is due to polarization of the spins ofthe conduction electrons in the metal, and has the greatest influence onthe position of the resulting metal NMR line in most bulk metals.

The Knight shift can be used advantageously to estimate the size of ametal nanoparticle and its interactions with ligands bound to itssurface, using a model which we now briefly describe. The percentage ofsurface atoms, and thus the size of the cluster, can be approximated bythe layer model. In the perfect case one assumes that the core of theTPPTP-Pt NP is comprised of platinum atoms arranged in an fcccubooctahedral pattern. This would suggest that there is a metal corecomposed of successive shells, or layers, of Pt atoms arranged around acentral Pt atom, consistent with the EXAFS observations. The number ofshells is determined by how large the metal core diameter is, i.e. themore shells involved, the larger the core diameter. According to thelayer model if the diameter of the particle core, “d”, is known thetotal number of atoms, “N_(t)”, involved can be approximated by eq. (3):

$\begin{matrix}{d = {a\sqrt[3]{\left( \frac{3N_{t}}{2\pi} \right)}}} & (3)\end{matrix}$

where “a” is the bulk lattice constant for a Pt atom (0.392 nm). Thus,the number of shells, “n”, revealing the number of surface atoms,“N_(s)”, can be calculated from eqs. (4) and (5):

N _(t)=( 10/3)n ³−5n ²+( 11/3)n−1  (4)

N _(s)=10n ²−20n+12  (5)

From these equations it can be found that the smallest possible clusterhas two shells; one central atom surrounded by a shell of 12 additionalatoms. This gives a total of 13 atoms, 92% of which reside on thesurface, and correlates to a 0.72 nm diameter particle. As the NP sizeincreases, the number of shells clearly also increases, with a 4 nmparticle having as many as 9 shells.

¹⁹⁵Pt NMR is a useful probe to distinguish between resonances due to thesurface the underlying atoms contained in the various Pt shells. Thisinformation can then be used to determine the dispersion, or thepercentage of atoms on the surface, and thus estimate the overallparticle size. When probing inward from the particle surface throughsuccessive layers, a point known as the “healing length” is eventuallyreached at which surface effects diminish and behavior approaches thatof the bulk metal. That is, when moving from the surface to the innershells the surface NMR peak position “heals” back to the bulk metal peakposition, as described by the exponential healing layer model shown ineq. (6):

$\begin{matrix}{K_{n} = {K_{\infty} - {\left( {K_{\infty} - K_{0}} \right)^{(\frac{- n}{m})}}}} & (6)\end{matrix}$

In eq. (6), “n” is the layer number starting at the NP surface (i.e.,n=0), “K_(n)” is the peak shift due to the n^(th) layer, “K_(∞)” is thebulk Knight shift (1.138 G/kHz), “K₀” is the surface layer Knight shift,and “m” is the number of layers defining the healing length whichincreases as the electronegativity of the atom bound to the surfaceincreases.

FIG. 11 depicts the room temperature solid state ¹⁹⁵Pt NMR of theTPPTP-stabilized Pt NPs acquired on a point-by-point basis and the fitto the data (solid lines) according to the healing layer model of eq.(6), with K₀, m, and the Gaussian peak widths as the fitting parameters.The non-symmetrical ¹⁹⁵Pt NMR line shape has been deconvoluted toseparate peaks, each with different shifts; those due to the surfaceatoms, S, and those due to the underlying sub-surface bulk atoms, B.From integrated areas of these peaks and using the above equations, weestimate a Pt NP having 3-4 shells with an effective size of ˜1.0 nm.This result is in agreement with the EXAFS experiments of Example 13 inthat it indicates that only the inner core of atoms resemble bulk metal,while the majority of the platinum atoms are non-metallic in nature dueto strong electronic coupling with surface-bound molecules. Thisbehavior is similar to the effect of CO adsorption to the surfaces oftransition metal clusters, which has been attributed to alterations inthe Fermi level local density of states (E_(f)-LDOS) of the metal uponbinding to the ligand.

The differences in TPPTP-Pt NP sizes observed between the ¹⁹⁵Pt NMRresults discussed in this example and the TEM image in Example 11reflects the distortions of the NP by the coordinated TPPTP ligand. BothEXAFS and solid state ¹⁹⁵Pt NMR analyses show a high percentage ofsurface to bulk Pt atoms reflecting this distortion. Consequently, theTEM, EXAFS, and ¹⁹⁵Pt NMR data are most consistent with a NP shapecomprising a flattened or truncated cubooctahedral geometry, withsignificant electronic interaction of the surface atoms with adsorbedmolecular species.

Example 15 Surface pK_(a) Determination for the TPPTP-Pt NPs

This example describes measurements of the pK_(a)'s of TPPTP-Pt NPselectrostatically adsorbed to indium tin oxide (ITO) or PAH-coated ITOsubstrate surfaces as model surfaces for multilayer fabrication.

Surface pK_(a) measurements were made according to the method of Liu,et. al. for TPPTP-Pt NPs electrostatically bound to model ITO andPAH-coated ITO electrode surfaces (J. Liu, L. Cheng, B. Liu, S. DongLangmuir, 16, 7471 (2000)). Briefly, an ITO electrode was treated with a0.01 M HCl/0.01 M NaCl aqueous solution containing 2 mg/mL PAH for 30min to provide a PAH-coated ITO electrode. Pt NPs were then bound toboth the PAH-coated ITO electrode and a bare ITO electrode by immersionin a 0.01 M HCl/0.01 M NaCl aqueous TPPTP-Pt NP dispersion (0.3 mg/mL)for 2 h. Currents associated with the reversible oxidation offerrocyanide (1 mM) in aqueous solutions having different pHs (adjustedusing HCl or NaOH with NaCl at 0.02 M constant total ionic strength)were measured and the pK_(a)'s extracted from the resulting current vs.pH curves. Values of pK_(a1) ˜2.0±0.2 and pK_(a2) ˜8.4±0.3 were obtainedfor TPPTP-Pt NPs chemisorbed via the phosphonate groups to the ITOsurface. Values for the PAH-coated ITO substrate of pK_(a1) ˜1.8±0.2 andpK_(a2) ˜9.0±0.3 differed only slightly from corresponding pK_(a) valuesobtained on the bare ITO surface. For a typical TPPTP ligand bearingthree phosphonate groups having a total of six P—O—H sites, theseresults indicate that even at pH 2 (e.g., and aqueous 0.01 M HCl/0.01 MNaCl solution), each TPPTP ligand should still possess an average chargeof approximately −1.5 units, which is sufficient to maintain stabilityof the Pt NP dispersion during multilayer fabrication.

Example 16 Demonstration of Reproducible Fabrication of UniformMultilayers Containing TPPTP-Pt NPs and PAH From pH 2 Solution on FusedSilica Substrates

This example demonstrates the ability to fabricate uniform multilayerscomprising alternating layers of TPPTP-Pt NPs and PAH on EDA-coatedfused silica slides using a dipcoating method.

EDA siloxane films were chemisorbed to clean fused silica slidesaccording to the literature procedure (M-S. Chen, S. L. Brandow, C. S.Dulcey, W. J. Dressick, G. N. Taylor, J. F. Bohland, J. H. Georger, Jr.,E. K. Pavelchek, J. M. Calvert J. Electrochem. Soc., 146, 1421 (1999)).Multilayer assemblies of TPPTP-Pt NPs were assembled via electrostaticLBL deposition with poly(allylamine hydrochloride) (PAH) on the fusedsilica slides coated with the cationic EDA monolayer film using theStratoSequence VI® robot dipcoater (nanoStrata Inc.) as described in thegeneral Examples section above. Deposition times for the TPPTP-Pt NPdispersion (0.3 mg/mL in 0.01 M HCl/0.01 M NaCl aqueous solution) andPAH solution (0.3 mg/mL in 0.01 M HCl/0.01 M NaCl aqueous solution) were2 h and 30 min, respectively. FIG. 12 shows the UV-visible absorptionspectra for fused silica (FS) slides bearing multilayer films having thestructure FS-EDA/TPPTP-Pt NP/(PAH/TPPTP-Pt NP)_(n-1), with n=6, 10, 16,and 20 on each side of the slide. The absorbance of the TPPTP-Pt NP/PAHbilayers was monitored by UV-visible spectrometry at 223 nm and 300 nmduring deposition, as shown in the inset of FIG. 12. The linearity in aplot of absorbance versus number of bilayers confirms uniform filmgrowth and deposition of the PAH and TPPTP-Pt NP layers. The absence ofplasmon resonance bands in the UV-visible spectrum indicates that thereis no aggregation (J. Schmitt, G. Decher, W. J. Dressick, S. L. Brandow,R. E. Geer, R. Shashidhar, J. M. Calvert Adv. Mater., 9, 61 (1997)) ofthe TPPTP-Pt NPs in the film and is consistent with the presence ofsub-3 nm particles in accordance with solid state NMR (Example 14), TEM(Example 11), and EXAFS (Example 13) analyses.

Example 17 Demonstration of Reproducible Fabrication of UniformMultilayers Containing TPPTP-Pt NPs and PAH From pH 2 Solution on GlassyCarbon Electrode Substrates

This example demonstrates the ability to fabricate uniform multilayerscomprising alternating layers of TPPTP-Pt NPs and PAH on anAPh-functionalized glassy carbon electrode using a dipcoating method.

GCEs modified with APh functional groups were prepared according to themethod of Example 9. Multilayer assemblies of TPPTP-Pt NPs were preparedanalogous to the method of Example 16 via electrostatic layer-by-layerdeposition with PAH on the protonated APh modified GCES. To assembleTPPTP-PT NPs/PAH multilayers, a protonated GCE-APh electrode was dippedinto an aqueous 0.01 M HCl/0.01 M NaCl dispersion containing theTPPTP-Pt NPs (0.3 mg/mL) for 24 h and rinsed with water, resulting in aone layer TPPTP-Pt NP electrode designated as GCE-APh/TPPTP-Pt NP.Additional TPPTP-Pt NP layers were added as required by sequentialimmersion in an aqueous 0.01 M HCl/0.01 M NaCl solution containing PAH(2 mg/mL) for 30 minutes, rinsing in water, immersion in aqueous 0.01 MHCl/0.01 M NaCl dispersion containing the TPPTP-Pt NPs (0.3 mg/mL) for24 h, and rinsing again in water. The composition of the electrode soobtained is indicated by the shorthand notation, GCEAPh/TPPTP-PtNP/(PAH/TPPTP-Pt NP)_(n-1), where n is the total number of TPPTP-Pt NPlayers present following n−1 PAH/TPPTP-Pt NP treatment cycles of theGCE-APh/TPPTP-Pt NP electrode. Separate electrodes were prepared havingfrom one (i.e., n=1) to 6 (i.e., n=6) TPPTP-Pt NP layers in this mannerand analyzed following use of each electrode in the electrochemistryexperiments described in the Examples below to determine Pt loading viaRutherford Backscattering Spectroscopy (RBS). The plot of Pt loadingversus number of TPPTP-Pt NPs layers (FIG. 13) shows that the platinumloading increases linearly with each layer deposited, in good agreementwith the UV-visible spectroscopy results for uniform deposition ofanalogous multilayers fabricated on FS slides in Example 16. Note thatneither the RBS plot in FIG. 13 nor the UV absorbance plots in FIG. 12(inset) intersect at the origin, resulting in a positive offset for eachline. This behavior indicates that larger amounts of TPPTP-Pt NPs aredeposited on both the APh and EDA films covering the respectivesubstrates than on PAH layers comprising the subsequent multilayer filmsunder these deposition conditions.

Example 18 Demonstration of Reproducible Fabrication of UniformMultilayers Containing TPPTC-Pt NPs and PAH From pH 2 Solution on FusedSilica Substrates

This example demonstrates the ability to fabricate uniform multilayerscomprising alternating layers of TPPTC-Pt NPs and PAH on EDA-coatedfused silica slides using a dipcoating method.

TPPTC-Pt NPs were prepared as described in Example 5. The experimentdescribed in Example 16 was then repeated using a dispersion of TPPTC-PtNPs (0.3 mg/mL) in 0.01 M HCl/0.01 M NaCl aqueous solution and PAH (2mg/mL) in 0.01 M HCl/0.01 M NaCl aqueous solution to fabricate aFS-EDA/TPPTC-Pt NP/(PAH/TPPTC-Pt NP)₁₉ multilayer on each side of anEDA-coated FS slide. Plots of film absorbance at 223 nm and 300 nm vs.the number of TPPTC-Pt NP layers deposited were both linear, consistentwith deposition of a uniform film containing non-interacting Pt NPs.Specifically, at 223 nm, we obtain a slope=0.0458, intercept=0.1066, andcorrelation coefficient (R²)=0.9996 for the absorbance vs. number ofTPPTC-Pt NP layers plot. Corresponding values at 300 nm are:Slope=0.0309; Intercept=0.1287; R²=0.9940. From Example 16, for TPPTP-PtNP/PAH multilayers prepared under identical conditions, we obtainSlope=0.0361; Intercept=0.105; R²=0.9992 at 223 nm and Slope=0.0244;Intercept=0.098; R²=0.9969 at 300 nm.

Example 19 Demonstration of Reproducible Fabrication of UniformMultilayers Containing TPPTS-Pt NPs and PAH From pH 2 Solution on FusedSilica Substrates

This example demonstrates the ability to fabricate uniform multilayerscomprising alternating layers of TPPTS-Pt NPs and PAH on EDA-coatedfused silica slides using a dipcoating method.

TPPTS-Pt NPs were prepared as described in Example 6. The experimentdescribed in Example 16 was then repeated using a dispersion of TPPTS-PtNPs (0.3 mg/mL) in 0.01 M HCl/0.01 M NaCl aqueous solution and PAH (2mg/mL) in 0.01 M HCl/0.01 M NaCl aqueous solution to fabricate aFS-EDA/TPPTS-Pt NP/(PAH/TPPTS-Pt NP)₁₉ multilayer on each side of anEDA-coated FS slide. Plots of film absorbance at 223 nm and 300 nm vs.the number of TPPTS-Pt NP layers deposited were both linear, consistentwith deposition of a uniform film containing non-interacting Pt NPs.Specifically, at 223 nm, we obtain a slope=0.0297, intercept=0.0291, andcorrelation coefficient (R²)=0.9958 for the absorbance vs. number ofTPPTS-Pt NP layers plot. Corresponding values at 300 nm are:Slope=0.0187; Intercept=0.0124; R²=0.9969.

Example 20 Demonstration of the Ability to Tune Composition ofMultilayers Containing TPPTP-Pt NPs and PAH by Varying the pH of theDeposition Solutions in Tandem

This example demonstrates the ability to alter the composition (i.e., PtNP loading) of multilayers comprising alternating layers of TPPTP-Pt NPsand PAH on EDA-coated fused silica slides by changing the pHs of thedeposition solutions in tandem.

The experiment described in Example 16 was repeated at pH 6.5, ratherthan pH 2.0, at equivalent solution ionic strengths (i.e., μ ˜0.02 M inboth cases) using a dispersion of TPPTP-Pt NPs (0.3 mg/mL) in 0.02 MNaCl (pH 6.5) aqueous solution and PAH (2 mg/mL) in 0.02 M MES (pH 6.5)aqueous buffer solution to fabricate a FS-EDA/TPPTP-Pt NP/(PAH/TPPTP-PtNP)₁₉ multilayer on each side of an EDA-coated FS slide. No plasmonresonance bands were observed in the UV absorbance spectrum of thecompleted multilayer film, consistent with deposition of a uniform filmexhibiting no Pt NP aggregation. The UV absorbances at 223 nm and 300 nmfor the completed film were ˜0.6106 and ˜0.4235, respectively.Corresponding absorbance values of ˜0.8250 at 223 nm and ˜0.5794 at 300nm were recorded for a film containing an identical number of TPPTP-PtNP/PAH layers prepared at pH 2 (i.e., 0.01 M HCl/0.01 M NaCl) accordingto the method of Example 16. Because absorbance is proportional to theconcentration of TPPTP-Pt NPs within the films, these differencesindicate that for films containing components whose net charge dependson the local pH, such as TPPTP-Pt NPs (note Example 15) and PAH, changesin pH of the solutions or dispersions containing each component caninfluence the Pt NP loading of the resulting multilayer film.

Example 21 Demonstration of the Ability to Tune Composition ofMultilayers Containing TPPTP-Pt NPs and PAH by Individually Varying theIonic Strengths of the Deposition Solutions

This example demonstrates the ability to alter the composition (i.e., PtNP loading) of multilayers comprising alternating layers of TPPTP-Pt NPsand PAH on EDA-coated fused silica slides by individually changing theionic strengths of the deposition solutions.

The experiment described in Example 20 was repeated with one variation.Specifically, the dispersion of TPPTP-Pt NPs (0.3 mg/mL) in 0.02 M NaCl(pH 6.5) aqueous solution was replaced by a dispersion of TPPTP-Pt NPs(0.3 mg/mL) in pure water (pH 6.5). Consequently, the NaClconcentration, and therefore the ionic strength of the dispersion used,was approximately zero. All other parameters remained the same as thoseused in Example 20 to fabricate a FSEDA/TPPTP-Pt NP/(PAH/TPPTP-Pt NP)₁₉multilayer on each side of an EDA-coated FS slide. No plasmon resonancebands were observed in the UV absorbance spectrum of the completedmultilayer film, consistent with deposition of a uniform film exhibitingno Pt NP aggregation. The UV absorbances at 223 nm and 300 nm for thecompleted film were ˜0.2706 and ˜0.1269, respectively. Correspondingabsorbance values of ˜0.6106 at 223 nm and ˜0.4235 at 300 nm wererecorded for the film containing an identical number of TPPTP-Pt NP/PAHlayers prepared at pH 6.5 using component solutions each having ˜0.02 Mionic strength according to the method of Example 20. Because absorbanceis proportional to the concentration of TPPTP-Pt NPs within the films,these absorbance differences indicate that Pt NP loading in thesemultilayer films can be controlled by individually adjusting the ionicstrength or the component solutions or dispersions used to fabricate themultilayer film.

Example 22 Demonstration of the Catalytic Activity of theGCE-APh/(TPPTP-Pt NP/(PAH/TPPTP-Pt NP)_(n-1) Multilayer ElectrodeArchitectures for the HOR

This example shows that multilayer electrode assemblies prepared usingTPPTP-Pt NPs and PAH exhibit high catalytic activities for the HOR atlow Pt loadings, similar to those noted previously for single crystalPt(110) (N. M. Marković, B. N. Grgur, P. N. Ross J. Phys. Chem. B, 101,5405 (1997)).

Voltammetry of the HOR for GCE-APh/TPPTP-Pt NP/(PAH/TPPTP-Pt NP)_(n-1)multilayer electrode architectures comprising one (n=1; inset: squares),two (n=2; inset: triangles), and three (n=3; inset: circles) TPPTP-PtNPs layers is shown in FIG. 14. The multilayer electrodes exhibit highactivity, indicating that the catalytic sites for hydrogen adsorptionare not blocked by the ligands and charge and mass transport throughthese films is not hindered. Further inspection of FIG. 14 shows thatthe anodic current rises sharply and reaches a plateau at relatively lowoverpotentials similar to Pt(110). At higher overpotentials, the currentis determined by the diffusion of H₂ through the acidic media, and thetheoretical value of the diffusion-limited current at the RDE, i_(d), isgiven by the Levich equation (eq. (7)):

i _(d)=0.62n _(e) FD ^(2/3) v ^(−1/6) c ₀ω^(1/2)  (7)

In eq. (7), n_(e) is the number of electrons exchanged in the reaction,D is the diffusion coefficient of H₂ in 0.1 M HClO₄ at 60° C., v is thekinematic viscosity of the electrolyte, c₀ is the bulk concentration ofH₂ in solution, and ω is the angular velocity of the RDE. A minimum of 2layers TPPTP-PT NPs, which corresponds to a platinum loading of 3.2μg_(Pt)/cm², is usually required to reach the theoreticaldiffusion-limited current density of 3.1 mA/cm² at 1600 rpm asdetermined from the Levich equation. The limiting currents at 2 and 3layer films of TPPTP-Pt NPs obey the Levich equation and increaselinearly with the square root of rotation rate from 100-2500 rpm (inset,FIG. 14).

The standard current-overpotential relation for a reaction under mixeddiffusion-kinetic control is described by eq. (8):

i/i _(o)=(1−i/i _(l,a))e ^(−αneFη/RT)−(1−i/i _(l,c))e^((1−α)neFη/RT)  (8)

In eq. (8), i is current, i_(o) is the exchange current, i_(l,a) andi_(l,c) are the limiting anodic and cathodic currents, respectively,n_(e) is the number of electrons involved in the slow electron-transferstep, α is the transfer coefficient for the reaction (typically ½), andη is the overpotential, alternately expressed as E-E°′, where E is thepotential and E°′ is the formal potential for the reaction. Atsufficiently large η, the cathodic term becomes negligible for the HOR,and equation 8 can be rearranged to yield the familiar Tafel form shownin eq. (9):

η=(2.303RT/αn _(e) F)log i ₀−(2.303RT/αn _(e) F)log i _(k)  (9)

In eq. (9), i_(k) is the kinetic current, which can be related to theTafel current by eq. (10) for a reaction under mixed kinetic-diffusioncontrol. If the rate-determining step is a slow electron transfer,plotting η versus the log term yields a straight line with a slope of59/αn mV at 298 K, or 66/αn mV at 333 K.

i _(k) =i·i _(l,a)/(i _(l,a) −i)  (10)

Tafel slopes of +44 and +32 mV/dec were calculated for 2 and 3 layerTPPTP-Pt NP electrodes, respectively, as shown in FIG. 15. At lowoverpotentials, Marković and Ross report Tafel slopes for the HOR onvarious Pt crystal faces: +28 mV/dec for Pt(110), +37 mV/dec for Pt(100)and +74 mV/dec for Pt(111). They also concluded that the differentcrystal faces utilize different reaction mechanism for the HOR. Whilethe TPPTP-Pt NPs display several crystal faces (note Example 12, FIG.9), the Tafel slope for the 3 layer film of TPPTP-Pt NPs indicates asimilar HOR mechanism as to Pt(110). We interpret the higher Tafel slopefor the 2 layer film of TPPTP-Pt NPs, not to a change in reactionmechanism, but rather to a Pt-limited reaction condition.

Example 23 Demonstration of the Catalytic Activity of theGCE-APh/(TPPTP-Pt NP/(PAH/TPPTP-Pt NP)_(n-1) Multilayer ElectrodeArchitectures for the ORR

This example shows that multilayer electrode assemblies prepared usingTPPTP-Pt NPs and PAH exhibit high catalytic activities for the ORR, withmaximum mass-specific current densities comparable to those obtained onPt-Vulcan carbon electrodes (H. A. Gasteiger, S. S. Kocha, B. Sompalli,F. T. Wagner Appl. Catal. B-Environ., 56, 9 (2005)) noted for themultilayer electrode comprising five TPPTP-Pt NP layers.

FIG. 16 a compares the anodic sweeps of the ORR voltammetry onGCE-APh/TPPTP-Pt NP/(PAH/TPPTP-Pt NP)_(n-1) multilayer electrodearchitectures comprising two (n=2), three (n=3), four (n=4), five (n=5),and six (n=6) TPPTP-Pt NPs layers at 60° C. At larger overpotentials(potentials more negative than ˜0.7 V for 4-6 layers of TPPTP-Pt NPs),the current is determined by the diffusion of O₂ through the acidicmedia, and the theoretical value of i_(d) for an RDE is given by theLevich equation (eq. (7)). A minimum of 4 layers TPPTP-Pt NPs, whichcorresponds to a platinum loading of 5.6 μg_(Pt)/cm², is required toreach the theoretical diffusion-limited current density of −6.0 mA/cm²at 1600 rpm, as determined from the Levich equation, using the kinematicviscosity of the electrolyte (v=1.009×10⁻² cm²/s), diffusion coefficientof oxygen (D=1.93×10⁻⁵ cm²/s), and the concentration of dissolved oxygenin solution (c₀=1.26×10⁻⁶ mol/cm³). Five layers of TPPTP-Pt NPs yieldsthe highest mass-specific activity, i_(m), at 0.9 V of 0.11 A/mg_(Pt)(FIG. 16 b) as calculated from the kinetic current, i_(k) (eq. (6)), andnormalized with the platinum loading of 6.6 μg_(Pt)/cm². This valuecompares favorably to values of 0.19 A/mg_(Pt) obtained using Pt-Vulcancarbon electrodes in HFCs (H. A. Gasteiger, S. S. Kocha, B. Sompalli, F.T. Wagner Appl. Catal. B-Environ., 56, 9 (2005)). As the amount ofTPPTP-Pt NPs layers is increased to 6 the mass-specific activity dropsto 0.09 A/mg_(Pt) at 0.9 V (FIG. 16 b). Although more platinum is on theelectrode surface in 6 layers of TPPTP-Pt NPs, a maximum current densityis reached with 5 layers. The source of this behavior is presentlyunclear. However, no electronic resistivity is evident in electrodevoltammetry performed in argon (note FIG. 17), indicating thatsignificantly increased electronic resistance for the 6 layer TPPTP-PtNP electrodes compared to the analogous 5 layer electrodes is not thecause of this behavior. Further investigations are underway to addressthis matter.

Example 24 Demonstration of the ORR Stoichiometry at theGCE-APh/(TPPTP-Pt NP/(PAH/TPPTP-Pt NP)_(n-1) Multilayer ElectrodeArchitectures

This example shows that multilayer electrode assemblies prepared usingTPPTP-Pt NPs and PAH can completely reduce oxygen to water by fourelectrons with high catalytic activities at low Pt loadings, similar tothose noted previously for single crystal Pt(111) (J. X. Wang, N. M.Marković, R. R. Adzic J. Phys. Chem. B, 108, 4127 (2004)).

The slope of a linear plot derived from the Levich equation, as shown inFIG. 18, can be used to determine the number of electrons involved inthe ORR. The experimental values of 0.44, 0.45, and 0.45(mA/cm²)rds^(−1/2), for 4, 5, and 6 layers of TPPTP-Pt NPs agree wellwith the complete reduction of O₂ by four electrons. Two and threelayers of TPPTP-Pt NPs had much lower slopes of 31 and 34(mA/cm²)rds^(−1/2), respectively, indicating that not enough activeplatinum was accessible at the electrode surface to reduce all availableO₂ to water.

FIG. 19 shows a logarithmic plot of the mass-transport correctedcurrents in the Tafel region for ORR at 4, 5, and 6 layer TPPTP-Pt NPelectrodes. It is clear from the plot that there are 2 distinct slopesat higher (E>0.85 V) and lower potentials (E<0.83 V) indicating that therate limiting step changes. The intrinsic Tafel slope for Pt with noadsorbates other than the ORR intermediates is −118 mV/decade (J. X.Wang, N. M. Marković, R. R. Adzic J. Phys. Chem. B, 108, 4127 (2004)).Deviations in the Tafel slope at higher potentials in perchloric acidare attributed to OH⁻ adsorption, which hinders oxygen reduction. Theexperimental values of −75, −67, and −71 mV/decade at higher potentialsfor 4, 5, and 6 layers TPPTP-Pt NPs, respectively, are consistent withresults seen for ORR on Pt(111) (Table 2). Correlations of Tafel slopesand voltammetry between well-defined single-crystal electrodes, such asPt (111), and Pt NPs suggest that ORR mechanisms are the same for eachelectrode.

TABLE 2 Kinetic Parameters for the ORR in 0.1 M HClO₄ at 60° C. onGCE-APh/TPPTP-Pt NP/(PAH/TPPTP-Pt NP)_(n−1) electrodes Mass- specificLayers of Tafel slope Tafel slope activity, Pt TPPTP-Pt 0.92–0.85 V0.83–0.73 V i_(m(9.0 V)) loading NPs (mV/dec) (mV/dec) (A/mg_(Pt))(μg_(Pt)/cm²) 4 −75 −109 0.03 5.6 5 −67 −104 0.11 6.6 6 −71 −101 0.088.1 Pt(111) −59 −118 — —

Example 25 Demonstration of the Stability of the GCE-APh/(TPPTP-PtNP/(PAH/TPPTP-Pt NP)_(n-1) Multilayer Electrode Architectures

This example shows that the GCE-APh/(TPPTP-Pt NP/(PAH/TPPTP-Pt NP)_(n-1)multilayer electrode architectures are sufficiently stable and maintaintheir catalytic activity during repeated use or after storage.

In order to confirm the stability of the multilayer electrodes duringour HOR and ORR experiments, a 5 layer TPPTP-Pt NPs electrode was cycledbetween 1.0 and 0 V at 20 mV/s for 20 cycles in 0.1 M HClO₄ at 60° C. Noappreciable loss in the catalytic current of oxygen reduction wasobserved, consistent with reproducible catalytic behavior. Additionally,no change in the mass-specific activity at 0.9 V was observed afterexposure to air for several days. Furthermore, XPS spectra werecollected on APh-grafted carbon paper modified with a monolayer ofTPPTP-PT NPs before and after electrochemical measurements to confirmthe TPPTP ligand remains intact. No measurable change was observed inthe Pt 4f or P 2p region after electrochemical measurements, indicatingthat the ligands remain unchanged.

Example 26 Demonstration of the Ability of Various Metal Ions toCrosslink TPPTP-Pt NPs

This example shows that various cationic metal ions in aqueous solutioncan effectively crosslink TPPTP-Pt NPs via interaction with the TPPTPligand phosphonate groups.

Aqueous solutions containing Fe(II)(NH₄)₂SO₄, Ca(II)Cl₂, Eu(III)(NO₃)₃,Ni(II)SO₄, Cu(II)Cl₂, and Co(II)SO₄ were prepared by separatelydissolving ˜5 mg of each salt in 1 mL water in individual test tubes. Adispersion of TPPTP-Pt NPs (0.3 mg/mL) in 0.01 M HCl/0.01 M NaCl aqueoussolution was prepared and 1 mL aliquots were pipetted into each of thetest tubes containing the solutions of the aforementioned metal salts.The mixtures were then observed during the next ˜24 h for bulkprecipitation of the TPPTP Pt NPs from the mixture, which served as anindicator for the interaction of the metal ions with the phosphonategroups of the TPPTP ligand. For the mixtures containing Fe(II)(NH₄)₂SO₄,Ca(II)Cl₂, and Eu(III)(NO₃)₃, a brown bulk precipitate formed within15-30 minutes, leaving a clear, nearly colorless supernatant consistentwith strong interactions of these ions with the phosphonate groups ofthe TPPTP ligand leading to extensive crosslinking. For the Cu(II)Cl₂,formation of the bulk precipitate required ˜30-60 minutes, consistentwith weaker interactions of this metal ion with the phosphonate groupsof the TPPTP ligand. Precipitate was formed for the mixture containingthe Ni(II)SO₄ only after 60 minutes and the total amount (i.e., volume)of material precipitated was only approximately half that observed forthe other metal ions discussed above. For the experiment usingCo(II)SO₄, precipitate was formed only after allowing the mixture tostand in air overnight. As a control experiment, the UV absorbancespectrum of the freshly-prepared TPPTP-Pt NP dispersion in 0.01 MHCl/0.01 M NaCl aqueous solution (pH ˜2.0) was compared with that of thesame dispersion aged 10 days at room temperature. No noticeabledifferences in the spectra were observed, consistent with the stabilityof the dispersion under these conditions (i.e., neither the HCl nor NaClpresent interact with the phosphonate groups sufficiently strongly todestabilize the dispersion to produce a bulk precipitate). Theseexperiments indicate that the interactions of metal ions with thephosphonate groups, and therefore their efficacy as materials forcrosslinking the TPPTP-Pt NPs for fabrication of multilayer electrodearchitectures, decreases in the approximate order:Fe(II)(NH₄)₂SO₄˜Ca(II)Cl₂˜Eu(III)(NO₃)₃>Cu(II)Cl₂>Ni(II)SO₄>Co(II)SO₄.

Example 27 Demonstration of the Ability to Reproducibly Fabricate aMultilayer Film Comprising Anionic Nafion® Ionomer and CationicPoly-2-Vinylpyridine by LBL Electrostatic Deposition

This example demonstrates the ability to fabricate multilayer filmscontaining polyanionic Nafion® ionomer using the LBL dipcoatingdeposition method.

An alcohol stock solution, useful for preparation of the Nafion®solution described below and as a rinse solution during multilayerfabrication, was prepared by mixing 10 mL isopropanol, 20 mL ethanol,and 20 mL methanol in a 100 mL volumetric flask and diluting to the markwith water. A stock Nafion® treatment solution, hereafter referred to as“Nafion®-A” solution, was prepared by weighing 0.55 grams of awell-shaken 10% weight Nafion® dispersion in water into a 25 mLvolumetric flask on the weigh pan of a balance. A 4.5 mL aliquot of 0.1M HCl (aq) solution was then added to the flask by pipet and the flaskwas diluted to the mark with the alcohol stock solution. The resultingclear, colorless Nafion®-A solution contains ˜40% mixed alcohols byvolume, ˜0.018 M HCl, and ˜2 mg Nafion® per mL of solution. Apoly-2-vinylpyridine (2-PVP) solution containing ˜1 mg 2-PVP/mL solutionwas separately prepared by dissolving 101 mg of 2-PVP in 100 mL of 0.02M HCl (aq) solution.

Multilayer assemblies of Nafion® and 2-PVP were fabricated using theaforementioned solutions via electrostatic LBL deposition on EDA-coatedfused silica slides analogous to the method described in Example 16. TheNafion® polyanion was deposited onto the cationic EDA-coated FS slidefirst for 30 minutes using an unstirred Nafion®-A solution. The treatedslide was then rinsed twice with the alcohol stock solution (30 secondrinses), followed by a single 30 second rinse with water. The surface ofthe slide was blown dry for 1 minute in a filtered stream of N₂ gas(liquid nitrogen boil-off) and immersed in the unstirred 2-PVP solutionfor 30 minutes. Thereafter, the slide was rinsed three times with water(30 second rinses) and blown dry with N₂ gas as described above. A UVabsorbance spectrum of the treated slide was recorded before thetreatment sequence was repeated. A total of three Nafion®/2-PVP bilayerswere deposited to fabricate a film having the structureFS-EDA/(Nafion®/2-PVP)₃ on each side of the FS slide. The absorbance ofthe Nafion®/2-PVP bilayers was monitored by UV-visible spectrometry atthe ˜260 nm absorbance peak associated with the pyridyl filmchromophore. The linearity in a plot of absorbance at ˜260 nm versusnumber of bilayers shown in FIG. 20 confirms uniform film growth anddeposition of the Nafion® and 2-PVP layers, demonstrating the ability toutilize Nafion® as a component of multilayer films.

Example 28 Demonstration of the Porosity of Naflon® Layers Adsorbed ontoan Oppositely-Charged Surface for Multilayer Fabrication

This example shows that a layer of the polyanionic Nafion® ionomeradsorbed to the cationic EDA sites on an EDA-coated FS slide issufficiently disorganized and/or permeable to allow penetration andinteraction of a small anionicFe(II)[4,7-(m,p-sulfonatophenyl)₂-1,10-phenanthroline]₃ ⁴⁻ complex withavailable EDA cationic N sites on the surface.

An ˜0.02 M solution of tetrasodiumtris-(bathophenanthrolinedisulfonate)iron(II) complex (i.e.,Na₄Fe(II)[4,7-(m,p-sulfonatophenyl)₂-1,10-phenanthroline]₃, note FIG.21), hereafter designated as “FePhenS⁴⁻”, was prepared in situ bydissolving disodium bathophenanthrolinedisulfonate (1.61 g, ˜3 mmol) andFeSO₄.7H₂O (0.278 g, 1 mmol) in an ˜3:1 molar ratio, respectively, in 50mL of ˜0.01 M HCl (aq) solution. A ˜0.1 mg Nafion®/mL solution wasprepared by the method of Example 27 using ˜0.025 g, rather than 0.55 g,of 10% weight Nafion® dispersion per 50 mL of the final solution.EDA-coated fused silica slides were treated for times ranging from 15 sto 15 min using the ˜0.1 mg Nafion®/mL solution and rinsed and dried asdescribed in Example 27. The treated slides were then immersed in theFePhenS⁴⁻ solution (ε_(282 nm) ˜1.38×10⁵ L·mole⁻¹·cm⁻¹; ε_(538 nm)˜2.05×10⁴ L·mole⁻¹·cm⁻¹), together with an EDA-coated FS control slidethat had not been treated with the ˜0.1 mg Nafion®/mL solution, for 60min. The slides were removed from the FePhenS⁴⁻ solution, rinsed threetimes with water, and dried in a filtered stream of N₂ gas (liquid N₂boil-off).

UV-visible absorbance spectra were recorded for each slide, as shown inFIG. 22. For the control EDA-coated FS slide corresponding to no Nafion®deposition (i.e., black line spectrum; “0 min” treatment in FIG. 22),electrostatic binding of the anionic FePhenS⁴⁻ to the cationic EDA sitesreadily occurs, as shown by the strong absorbance peaks at ˜282 nm and538 nm corresponding to the FePhenS⁴⁻ in FIG. 22. For EDA-coated FSslides pre-treated with the Nafion® solution, significantly lessFePhenS⁴⁻ is bound. As Nafion® solution treatment times are increased,Nafion® coverage on the EDA surface increases and the quantity ofFePhenS⁴⁻ adsorbed decreases. The data in FIG. 22 clearly indicate thatbinding of the Nafion® film to the EDA-coated FS slide is essentiallycomplete within 15 minutes. In addition, the observation of FePhenS⁴⁻ onthe surface even after completion of the Nafion® binding indicates thatthe deposited Nafion® is sufficiently disorganized, porous, and/orconformationally labile such that the small FePhenS⁴⁻ species canpenetrate the film and interact with the underlying EDA coating, despiteelectrostatic repulsions with the anionic Nafion® layer.

Example 29 Demonstration of the Use of pH to Control Deposition ofAdjacent TPPTP-Pt NP and Nafion® Layers for Multilayer Fabrication on SiSubstrates Modified by Chemisorbed Aminophenylsiloxane Monolayers

This example shows the pH can be used to control the reactive amounts oflike-charged TPPTP-Pt NPs and Nafion® ionomer on APh-modified Si wafersas adjacent layers for the fabrication of multilayer electrodeassemblies.

A self-assembled monolayer of p-aminophenyltrimethoxysilane (APhS) waschemisorbed onto a SF slides and the native oxide layer of a Si wafer(as a surrogate for the APH-functionalized GCEs) by immersing a clean FSslide and Si wafer in a 1% volume solution of APhS in toluene: methanol95:5 v/v containing 1 mM acetic acid at 55° C. for 1 hr. The treatedsubstrates were rinsed twice with toluene, blown dry in a filtered N₂gas stream (liquid N₂ boil-off), and baked at 120° C. for 4 minutes tocomplete the chemisorption process. The resulting APhS film on the FSslide exhibited a UV absorbance of 0.065 at 221 nm, consistent withchemisorption of a monolayer film (i.e., monolayer absorbance estimatedusing ε_(221 nm)=55,400 L·mole⁻¹·cm⁻¹ and ε_(251 nm)=28,900L·mole⁻¹·cm⁻¹ in ACN). A Nafion® solution, hereafter designated“Nafion®-H” solution, was prepared as described in Example 27 for“Nafion®-A” solution with one variation: the 4.5 mL aliquot of 0.1 M HCl(aq) solution was replaced by a 2.5 mL aliquot of 0.1 M HCl (aq)solution and 2.0 mL aliquot of water to yield a final solution having˜0.01 M HCl (aq). A second Nafion® solution, hereafter designated“Nafion®-OH” solution, was also prepared as described in Example 27 for“Nafion®-A” solution with one variation: the 4.5 mL aliquot of 0.1 M HCl(aq) solution was replaced by a 4.5 mL aliquot of water to yield a finalsolution having no added HCl (aq).

Two pieces of the Si wafer bearing the chemisorbed APhS siloxanemonolayer were treated 2 hours with a TPPTP-Pt NP dispersion (0.3 mg/mL)in 0.01 M HCl/0.01 M NaCl aqueous solution, rinsed twice with water, anddried in a filtered N₂ gas stream (liquid N₂ boil-off). One of the Siwafer pieces was then treated 30 min using the Nafion®-H solution andthe other was treated 30 min using the Nafion®-OH solution. Each treatedwafer was rinsed with alcohol stock solution from Example 27 and driedin a filtered N₂ gas stream (liquid N₂ boil-off). The experiment wasrepeated with two additional pieces of Si wafer using a 24 hour, ratherthan 2 hour, treatment with the same TPPTP-Pt NP dispersion.

Each of the treated wafers was then analyzed by XPS to determine therelative amounts of TPPTP-Pt NPs and Nafion® ionomer film deposited ontothe surface, as measured by the relative areas (corrected for variationsin instrument sensitivity as a function of element/energy) of the Pt 4fand F 1s peaks. Table 3 summarizes the treatment conditions and relativeamounts of platinum and fluorine observed for each piece of Si wafer.Analysis of the results in Table 3 shows that use of the Nafion®-OHsolution, which contains no added HCl, leads to minimal deposition ofNafion® onto a TPPTP-Pt NP layer, especially for the Si wafer treatedonly 2 hour by the TPPTP-Pt NPs. While the effect is much lesspronounced for wafers treated 24 hours with the TPPTP-Pt NPs, the levelof platinum present is still ˜4.6 times that of fluorine. In contrast,use of the acidic Nafion®-H solution leads to deposition of Nafion® inamounts more nearly equivalent to the level of Pt present on thesurface. Specifically, platinum levels are only ˜1.7 times those offluorine on the surface, regardless of TPPTP-Pt NP treatment times used.This is clearly illustrated in FIG. 23, which shows XPS results for theAPhS-coated Si wafer treated 24 hours with the TPPTP-Pt NPs (i.e., 0.3mg/mL dispersion in 0.01 M HCl/0.01 M NaCl), followed by Nafion®-Hsolution (i.e., containing added ˜0.01 M HCl (aq)). These resultsclearly indicate that the relative ratios of TPPTP-Pt NPs and theNafion®ionomer deposited as adjacent layers on a surface can becontrolled by judicious choice of the pH of the Nafion® solution.

TABLE 3 XPS Results of TPPTP-Pt NP/Nafion ® Layer Depositions onAPhS-coated Si Wafers Nafion (HCl) Pt Dip Time Pt (%) F(%)  0.0 M  2 hr98.6 1.4 0.01 M  2 hr 1.7 1.0  0.0 M 24 hr 4.6 1.0 0.01 M 24 hr 1.7 1.0

Example 30 Demonstration of the Use of pH to Control Deposition ofAdjacent TPPTP-Pt NP and Nafion® Layers for Multilayer Fabrication on SiSubstrates Modified by Chemisorbed EDA Monolayers

This example shows the pH can be used to control the reactive amounts oflike-charged TPPTP-Pt NPs and Nafion® ionomer on EDA-modified Si wafersas adjacent layers for the fabrication of multilayer electrodeassemblies.

The experiments described in Example 29 were repeated using EDA-coatedSi wafers, rather than APhS-coated Si wafers, as substrates. The resultsare summarized in Table 4. Although the results generally parallel thosefor the APhS-coated Si wafers in Example 29, there are some differences.Specifically, in contrast to the results shown in Table 3 for theAPhS-coated Si wafers, differences in relative quantities of TPPTP-PtNPs and Nafion® ionomer deposited onto the EDA-coated substrates areessentially independent of TPPTP-Pt NP treatment time for depositionsusing the Nafion®-OH solution. In addition, for the experiments usingNafion®-H solution, the treatment time of the substrate with theTPPTP-Pt NP dispersion more strongly affects the subsequent depositionof the Nafion® layer. These results indicate that the nature of theunderlying film (i.e., in this case the monolayer chemisorbed to the Sisubstrate) also influences the relative amounts of TPPTP-Pt NPs andNafion® that can subsequently be deposited in adjacent film layers.

TABLE 4 XPS Results of TPPTP-Pt NP/Nafion ® Layer Depositions onEDA-coated Si wafers Nafion (HCl) Pt Dip Time Pt (%) F(%)  0.0 M  2 hr97.0 3.0 0.01 M  2 hr 0.23 1.0  0.0 M 24 hr 96.0 4.0 0.01 M 24 hr 1.11.0

Example 31 Demonstration of the Ability to Control the ORR UsingMultilayer Electrodes Containing Adjacent TPPTP-Pt NP and Nafion® Layer

This example shows the fabrication of multilayers containing PAH,TPPTP-Pt NPs, and Nafion® as component layers can affect the ORR.

A GCE was chemically modified with APh functional groups according tothe method described in Example 9. The resulting electrode was then usedas a substrate for the fabrication of a multilayer electrode assembly ofstructure GCE-APh/TPPTP-Pt NP/(Nafion®/PAH/TPPTP-Pt NP)₃ analogous tothe method of Example 17. Specifically, TPPTP-Pt NP and PAH layers weredeposited using the same TPPTP-Pt NP and PAH solutions, rinseprocedures, and treatment times described in Example 17. For depositionof the Nafion® layers, the Nafion®-H solution, rinse procedures, and the30 minute treatment time described in Example 29 were used. Theresulting electrode was used to catalyze the ORR as described inExamples 23 and 24. FIG. 24 shows the ORR voltammetry of theGCE-APh/TPPTP-Pt NP/(Nafion®/PAH/TPPTP-Pt NP)₃ electrode (Curve “a”),together with results for the corresponding GCE-APh/TPPTP-PtNP/(PAH/TPPTP-Pt NP)₃ electrode from Examples 23 and 24 (Curve “b”). Forthe GCE-APh/TPPTP-Pt NP/(PAH/TPPTP-Pt NP)₃ electrode, the oxygenreduction current noted at 0.9 volts is −0.163 mA/cm², with a total Ptloading of ˜5.6 μg/cm² determined by RBS analysis. Addition of theNafion® layers in the GCE-APh/TPPTP-Pt NP/(Nafion®/PAH/TPPTP-Pt NP)₃electrode is accompanied by a slight increase in Pt loading to ˜6.9μg/cm². However, the over-potential for the ORR has now increased by ˜50mV (as shown by the shift of the voltammogram in Curve “a” to the leftof that shown in Curve “b” in FIG. 24), with a corresponding decrease ofthe oxygen reduction current to +0.031 mA/cm². This behavior indicatesthat the ORR has been suppressed by the inclusion of the Nafion® layers,which are well-known blockers of electron-transfer reactions.Consequently, the use of Nafion® layers as components of our multilayerelectrodes in the manner described in this example provides a convenientmeans to control the ORR, in this case suppressing the ORR relative tosimilar electrodes not containing Nafion® layers.

Example 32 Preparation of PAH Solutions Containing PerchlorateCounterions

This example describes the exchange of perchlorate ions for chlorideions in PAH solutions.

A known volume of 20 mg PAH/mL aqueous solution was prepared bydissolving PAH (average molecular weight range 8,000 g/mole, lot#TG123713MG) in water. Sufficient NaOH was added to bring the solutionto pH ˜12. The basic solution was then transferred into a Spectra/Pore®Biotech Cellulose Ester (CE) dialysis membrane (molecular weight cut-off500 g/mole; flat width ˜16 mm; diameter 10 mm; volume/length ˜0.81mL/cm, Spectrum Labs, Inc.) and dialyzed against water for 1 hour. Thedialysis tube containing the PAH sample was transferred to a containerof fresh water and dialysis was continued for an additional two hours,after which the tube was transferred to fresh water again and dialysiscontinued for an additional 24 hours. Finally, the dialysis tube wasagain transferred to a container of fresh water and dialyzed for anadditional 24 hours. The tube was then opened, the were contentstransferred to a flask, and diluted ˜1:10 v/v with an aqueous solutioncontaining sufficient HClO₄ and NaClO₄ to obtain a final solutioncontaining 2 mg PAH/mL in 0.01 M HClO₄/0.01 M NaClO₄ aqueous solution.

1. A method of making a nanostructured electrode comprising: a.electrochemically modifying a conductive glassy carbon electrodesubstrate to create a surface functional group capable of supportingmultilayer growth via covalent grafting of a functional group to createan initial layer of positive charge on the surface; b. depositing aplatinum nanoparticle stabilized by negatively-charged ligands onto saidfunctional group; and c. providing a polymer component.
 2. The methodaccording to claim 1 wherein said step of depositing said platinumnanoparticle stabilized by negatively-charged ligands onto saidfunctional group is repeated.
 3. The method according to claim 1additionally including rinsing said conductive glassy carbon electrodesubstrate after said step of depositing said platinum nanoparticlestabilized by negatively-charged ligands.
 4. The method according toclaim 3 wherein said rinsing is with DI water and further including astep of drying said conductive glassy carbon electrode substrate in astream of gas.
 5. The method according to claim 4 wherein said stream ofgas is nitrogen or air.
 6. A method of making a nanostructured electrodecomprising: a. depositing a self-assembled monolayer on a substrate; b.depositing a catalyst nanoparticle covalently bonded to a ligand; and c.depositing a material capable of binding to said self-assembledmonolayer.
 7. The method of claim 6 wherein said self-assembledmonolayer includes a surface functional group which creates a layer ofcharge or chemical reactivity on the surface.
 8. The method of claim 6wherein said ligand has a peripheral functional group that has a chargeopposite to or chemical reactivity amenable with that of saidself-assembled monolayer.
 9. The method of claim 7, wherein saidcatalyst nanoparticle is a platinum nanoparticle and said step ofdepositing said catalyst nanoparticle is repeated.
 10. The method ofclaim 9 further including repeating said step of depositing apolyelectrolyte having a charge opposite that of said ligand-stabilizednanoparticle.
 11. A method of making a nanostructured electrodecomprising: depositing on a conductive electrode substrate a catalyticnanoparticle stabilized by a covalently-bound ligand bearing aperipheral functional group; and depositing a material capable ofbinding to said peripheral functional group, wherein said conductiveelectrode substrate is chemically modified to create a surfacefunctional group capable of supporting multilayer deposition.
 12. Themethod of claim 11 wherein said material capable of binding to saidperipheral functional group is such that successive layers of saidcatalytic nanoparticle within a multilayered system are bridged by saidmaterial.
 13. The method of claim 12 wherein said material is selectedfrom the group consisting of semiconductors, RuO₂, ITO, and TiO₂. 14.The method of claim 12 wherein said bridging material is a surfaceoxidized carbon colloid.
 15. The method of claim 12 wherein saidbridging material is a polyoxometalate.
 16. The method of claim 12wherein said bridging material is a metal ion.
 17. The method of claim10 wherein said material is polyallylamine hydrochloride and saidcatalyst nanoparticle is a platinum nanoparticle having covalently boundnegatively-charged tris-[4-phosphonatophenyl]phosphine ligands.